aww man I have to say your in a bit of a pickle.
In a fish tank you have a filter which eventually grows bacteria, the bacteria is a naturally occurring one that breaks down fish waste and chemicals that are created as a result. This is called the nitrogen cycle, the easiest way to explain it is like this, fish live in their own toilet and when they poo the fish poo gives of a chemical called ammonia. Ammonia is highly toxic to fish and will cause major issues for you in the near future. Ideally we like to recommend people do a fishless cycle which by passes the fish poo and just used bottled ammonia but the situation your in now is called a fish in cycle.
So yeah the nitrogen cycle goes like this
fish poo -> ammonia -> nitrite - nitrate
to get from ammonia to nitrite you need a colony of bacteria to build up over a number of weeks but until that point the water is highly toxic to the fish, the good news is it will get better the bad news is nitrite is still toxic for fish but an other type of bacteria will grow along with the first type to make it into harmless nitrate then we as fish keepers remove the nitrate by doing water changes which should be done weekly not monthly.
Right now you are in a fish in cycle, even though the filter had fish in at some point with your brother in law the bacteria will have been starved as there was no ammonia source for a number of months now. So this means that the fish are pooping and the ammonia is being created but there is nothing in the filter to remove the ammonia see what I mean? So the best bit of advice now is to read the secion in the begginers resource section and learn what a fish in cycle is. This will mean you need to get an API master test kit with ammonia and nitrite testing kits so you can test daily for a while, alongside the water tests you will need to do daily water changes to keep the ammonia and nitrite below .25 ppm in the water.
Hope that has helped like I say check out the begginers resource section there is lot in there written by fishkeepers that you wont get told in the shops or most books! We have a lot of true experts on this forum that work within the aquatics trade and those are the ones that have written these articles. You will quickly learn that there are a lot of common myths in the hobby and as much as I hate to say it you have been told quite a few.
Now onto the next problem..... Cichlids, catfish and plecs are sometimes hardy fish, those three groupings of fish probably cover a few hundred thousand fish though and some are hardier than others. As for fish that get too big for your tank, the one i pointed out as a frontosa like you say will outgrow your tank but so will your plecs, the ones in the pics are common plecs which will grow to 10 inches in a year and then keep growing to between 12 inches to 16 inches. Like I say I am no expert with African Cichlids, my specialism is more with American Cichlids which is a whole different type of fish but I do know that most africans need large tanks but I believe that you may get away with this size tank, to get possitive IDs on these fish maybe post your pictures in the african cichlid section and people will be able to help you work out what you need to do with them and which ones are going to be able to live in your tank properly.
Hope thats helped somewhat, I think perhaps the best thing you could do is rehome the fish that are going to outgrow your tank and possibly at the same time try to take back the other fish so you can start a fishless cycle? Its much easier than the daily water changes a fish in cycle requires the other advantages of a fishless cycle is 0 chance of fish dieing in the cycle stage and also 0 chance of disease which can in some cases infect the tank and kill off future fish as well. The other thing to mention is that in low ph and high ammonia and nitrite I think your cichlids could be under extreme stress which can lead to disease. Dont get me wrong we have all made similar mistakes at some point but unfortunatly that is why you hear of so many people having a tank for a month or so and then giving up, because they didnt have access to this kind of information then all their fish die and they give up but hopefully the forum will be able to guide you through this rough start to a happy long lasting tank
Wills
do you have a water test kit? The liquid drop one? They are the most reliable well worth getting. 
