Sorry to answer a question with a question but what are you trying to achieve with the breeding? I can't work out what the aim is.
You're better off seperating the ones with the traits you desire and linebreeding (basically inbreeding) to 'set' the traits. If you're randomly breeding all sorts together it may provide you with a vigorous, healthy population but they may be quite drab in appearance.
if you're simply trying to cross species to see what the results are then you're going to have to seperate them so that you always have different species male and females together as they will always TEND to breed with their own type.
I've also heard that Molly/Guppy hybrids are almost always infertile so that may be a dead end there.
Swordtails and Platies are very similar and regularly hybridise so no problem there but there's also not much to be gained from the hybrid in terms of difference in shape/colour/finnage.