Right. First off, are you prepaired to return all the fish and start over? This is realy the best thing for both you and the fish.
What size tank is this BTW?
Now, before I start my (hopefuly informative) rant, I just wanted to tell you that, actualy, goldfish and tropicals SHOULD be treated in exactly the same manner excluding water temperature. The only reason people manage to mis-treat goldfish is because of the various myths surrounding their keeping, a lot of misinformation and the unfortunate fact that goldfish are exceptionaly hardy. It isn't usualy the goldfish-keepers fault, mind, but the fault of the people who sell them and don't tell you about their true needs. A single goldfish requires 20 gallons of water to itself. They like to be in groups. They get to 12" and they can live to be 30 years old. All the stuff abotu them 'growing to the size of the tank' and so on is just plain nonsense.
Anyway, I'm sure this doesn't realy concern you right now
so I'll go on with the 'rant':
You MUST get your own test kit. This realy is essential. It isn't very expensive and will last a long time. The liquid-based ones are best. Those strip-type ones with the paper that changes color or whatever need to be replaced regularly and aren't accurate. Avoid them despite the cheaper price - they will be a loss in the long run.
The reason you need the test kit is to monitor your ammonia, nitrIte and nitrAte levels. You see, right now, your tank is going through a 'cycle'.
The cycling process is basicaly this: Fish produce waste, waste contains ammonia, ammonia is converted to nitrItes by good bacteria, nitrItes are then converted to nitrAtes by some other good bacteria. Once the process is complete, you do weekly partial (20-25%) water changes with de-chlorinated water to remove the nitrAtes.
The reason this cycle is so important is that ammonia and nitrIte is incredibly toxic. Right now, your fish are being poisoned by their own waste. NitrAte is less harmful but still should be removed regularly to keep it below 40ppm. Better still, keep it under 20ppm by doing water changes at least weekly, not over-feeding and not over-stocking.
Now I'm going to tell you about the 'good bacteria' that convert all the harmful stuff to nitrAtes. They lvie in the gravel and in your filter media. Chlorine kills them. For these reasons, de-chlorinator is essential or your tap water, with all the chlorine it has in it, will kill off these good bacteria as well as irritating your fish. Also, the fact that most of teh good bacteria live in your filter media (the spongy stuff that 'filters' out any debris), means you shouldn't replace the media regardless of what the instruction booklet claims. If your filter gets clogged up, take the media out, rinse it in old tank water taken out from a partial water change. Then put it back in the filter.
If ever you realy need to replace it (seriously - only if it starts to disintegrade), only take out half at a time and leave a period of one week in between so the bacteria get to colonise the new media. Honsetly though, you don't need to worry about this just yet.
You can detect the cycle happening by testing your water. First, ammonia 'spikes' at its highest point as the fish waste accumulates. Then, good bacteria start to grow which change the ammonia to the nitrItes I talked about before. The ammonia will now be dropping and the nitrIte rising. It will soon 'spike' as well. Then more good bacteria grow that result int eh lowering of nitrItes and a 'spike' in nitrAte. Atrictly speaking, the nitrAte 'spike' isn't a 'spike' as it would increase indefinately if you didn't do water changes. This whole process will take a couple of weeks if you cycle without fish (using pure ammonia), 6 weeks or more if you cycle with fish as you are now attempting to do.
Now, as to what you are going to have to do for the next few weeks, possibly longer:
Do a water change daily. Do one today and for every day following until your ntirItes start to fall. These water changes will prolong the process but the fact is that the fish, particularly the mollies, won't make it otherwise. Something you can do to speed up the process, possibly even make it instantaneous if you are lucky, and which I strongly suggest you do is to ask your LFS for some used filter media and/or gravel. By 'used' I mean media right out of one of their tanks, that has fish in it, from inside one of the working filters. This media will contain a lot of the good bacteria. Take it home quickly (to minimize the number of bacteria lost in transit) and put it in your own fitler (instead of a piece that was in there already). If you can get this media today or tomorrow, you may just be able to save the fish. Using gravel in a similar way can also help - get some gravel (as much as possible within reason) from inside a tank at your LFS and put that in your tank. It will have the same effect as the filter media. Regardless of which method you go for, make shure the filter media/gravel is coming out of a tank that looks healthy - there shouldn't be any sick or dead fish in the tank.
A few more things I suggest you get while you are at your LFS: The used gravel/media obviously, a liquid-based test kit that includes ammonia, nitrIte and nitrAte (pH is useful to know as well), a gravel vacuum if you don't have one already (once your tank is cycled, you should use it for your weekly water change - while it is still cycling, however, you won't want to use it and mix up the gravel where the bacteria are beginning to colonise just yet). Also, I strongly suggest you buy some frozen foods - bloodworms and brine shrimp are great.
Also(from your local supermarket this time
), get yourself some frozen peas - the typical green ones. For the next week, you'll be taking a pea every day, microwaving/boiling till the 'skin/shell' can be removed (takes about 10 seconds to soften it up in a microwave), de-shelling it, crushing it and then feeding it to your fish. Any leftovers after about an hour should be removed or they'll rot and add to the already high ammonia levels. The pea may or may not work wonders for your molly. It sounds like it has a swim bladder problem but there's realy little else you can do for it at the moment.
Make sure you are only feeding once a day and doing your 25% daily water change about an hour afterwards. If you see algae growing in your tank BTW, don't remove it. It will actualy help absorb ammonia/nitrIte/nitrAte and will help keep these harmful chemicals' levels down now that it is so important (live plants have a similar effect but leave that for now). Once the tank is safe, you can scrape the algae off for aesthetic reasons.
I think that's it. I just want to remind you to tell me what size tank you have and whether you would be prepaired to return the fish. This realy would be the best option to be honest. In fact, if your tank is small, you may have to return a few fish anyway. If you do agree to return all of them, a fishless cycle will be very quick to do on your tank and it can also be sped up with used filter media/gravel like I described before. How to do a fishless cycle is pretty much covered in the link in my signature. There;s also some other informative articles I have linked to there so do read them. They basicaly cover what I have written but in more detail and there are some useful tips.
Oh, one more thing. I don't want you to do anything about this until after the tank is cycled but, zebra danios should be in groups of 5 at least. Also, mollies and platies are livebearers. the general rule for these fish is that you should have 2 females for every male. Otherwise, they should all be the same sex. The reason for this is that the males will chase any females and can seriously stress them out to the point at which they become ill and die. By having 2 females for every male, this chasing behaviour and the attention of the male(s) is spread out. You can tell what sex your fish are by looking at the anal fin. That's the fin udner the body just before the tail and behind the pair of fins under the body called the 'ventral fins'. If the anal fin is fan-shaped, the fish is female. If it is pointed and tube-like, the fish is male and what you are seeing is a 'gonopodium' - a modified fin used to deposit sperm packets directly into the female. The female then gives birth to lvie young. You should also eb aware that livebearers breed like crazy and that, even if you only have females, they can still produce fry (babies) as they can store sperm for several months. Luckily (in a strange way), most fry will get eaten. However, if there are enough hiding places, some will survive. This is ok in a large, cycled tank, but not good in one that is still cycling and unstable. To guarantee that you won't get fry, all your livebearers have to be male.
There's also an 'inch epr gallon guideline' that many fish-keepers use to determine the number of fish you can keep in a tank. It only applies to small fish and, even then, may not apply, but, in your case, it may prove a useful guideline. Basicaly, the guideline states that for every inch of ADULT fish (that is you use the potential adult size of the fish), there should be one gallon of water in your tank. Mollies get to 4" (sometimes more), platies to 1.5" and zebra danios to 1.5". So, by the guideline, you need a 15 gallon tank at least. However, danios are one of the fish this doesn't apply to as they need a 20 gallon minnimum due to activity levels. If you have anything smaller than that, you need to return your danio. If you have a 10 gallon, you should take back the mollies and the danio. The platy would be fine though and platies also happen to be very hardy so may be good to cycle with if you decide you don't want to fishless cycle after all.
Yes, I think I'm done now.
Welcome to the forum! I'm sure others have posted by now but nevermind... Don't be too hard on yourself - we all make mistakes, you obviously want to rectify them and it isn't your fault you didn't get any good advice originaly. Good luck!
It will take a good few weeks for the tank to "cycle". If you'd like to understand more about the Nitrogen Cycle, read 
you will find sooner rather than later you need a better filter better lighting and so on and so on, take your time and be patient with the process, take as much advice from the pro’s on here, and in no time I will be good! Chin up! 
it might well not be. I recommend you check properly and remove it asap if it's dead. If dead, wrap the fish in some paper towel and dispose of in kitchen waste bin. Do not flush down the toilet.
