First off, welcome to the forums.
Platies and Swordies and Guppies and Mollies all have different lengths of pregnancies and fry numbers...so I'll list them.
Mollies:
Mollies can have fry every 4-8 weeks, approximately. Some Mollies can have over 100 fry in each pregnancy, depending on size, and age. Obviously the bigger the female, the more fry she will have. They also have big fry.
Platies:
These cute little fish can have a lot of fry too. They can have around 70-90 max fry each month, depending on if you have Maculatus or Variatus. Variatus Platies grow bigger, and can therefore have more fry. However, you do not usually get the numbers you get from Mollies or Swordtails.
Swordtails:
These are also pretty big fry producers. These fish get to bigger sizes, much like Mollies. There have been accounts of well over a hundred fry from these fish, but this of course depends on age and size. As a note, Swordtails can be pregnant for up to 2 months, so don’t worry if a month goes by and you don’t have fry!
Guppies:
These fish are probably the biggest fry producers in the common Livebearer world. Their nickname isn’t Millions Fish for nothing! They can have big amounts of fry, every 4 weeks. I’m going to say the max is 100, but it may be more.
As you can see, all your fish are different. Signs of labor usually include being moody, chasing fish away, hanging around in the plants, slightly heavier breathing, looking squarish when viewed head on, and looking a little thinner then they did before.
I'd just let the mother give birth in the fry tank, and then take her out and raise the babies in there. Breeding boxes can stress them out and cause birthing complications or even abortions. To set up your fry tank for maximum results...
Temperature is good anywhere from 75-78 F or so. I think a sponge filter is best for a fry tank, so that they can't get sucked up. It's worked well for me in the past, so I would recommend it. Otherwise, you could use a HOB, or other filter and just cover the intake with a breeder net, or Pantyhose, or something similar so they don't get sucked up.
Ornaments can be anything you want...Plants are good, as are rocks, wood and general places for them to hide. Thick plants seem to be afavorite with mine. They can disappear into there, and fool their brothers and sisters as to where they are! Sand is the best substrate. With gravel, they can sometimes get stuck in between the pieces and get crushed. Bare bottom also works, and some find it easier to clean.
Fry need good water quality, and they are sensitive so lots of water changes! Daily ones are okay, once every 2 days is okay too. Just make sure the tank is always clean. You can do water changes using a airline tubing siphon you make, or a turkey baster so you don't suck up the little babies. Acclimation to the tank is impotant too. Take a loooong time...and they'll be better off.
Also, fry need to be fed often! 3 times a day at least, more if you can. When they are first born, Tetramin Fry Food or Hikari First Bites are best for them. Baby Brine Shrimp is good suppliment to this, or you can just feed that but it's best to vary. Once they grow up a bit, you can add in finely crushed flakes and Freeze-dried, frozen, or Live foods, such as Bloodworms, Brine Shrimp, Tubifex Worms..etc. However, don't feed them just this. These are all high protein foods, and they can cause them to be constipated or get bloated. Fry are sensitive. You can also try small pieces of blanched veggies, like cucumber, zucchini, peas...etc.
Sorry if I carried on too much.
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I have had a batch of ballon molly fry and have since set up a tank just for the babys i also have 3 baby platies. I thinks all are pregnent again, mollys and gupys but some one told me in the aquatic shop that they only take about a week and a half to have there fry
