Ask yourself this: do you want fry that are a specific variety ("pedigree") or a bit of this, bit of that ("moggie"). If you want to breed fish you can sell easily, you want to do the former. Fish shops don't really want mongrel platies or any other kind of fancy fish.
One of the best kept secrets in fishkeeping is that 10 of one fish looks ten times better than 10 different fishes. Look at a tank with 6 wild-type angels or 20 neons and you'll see what I mean. Large numbers of similar fish that school together look amazing. Individual fishes that all have clashing, contrasting colours tend to look like a jumble.
In a 50 gallon tank you could get, say, ten platies of a single variety, and then mix them with different species of fish so that any platy fry you got would be same variety as the parents. Ideally choose fish from different general (i.e., not swordtails or other Xiphophorus spp.). Sunset platies with black mollies, glassfish, wrestling halfbeaks, orange chromides, and bumblebee gobies for example would give a nice mix of fishes of totally different shapes and colours and swimming levels. They'd all do well in hard, alkaline water with a little salt added for the benefit of the mollies, chromides, and gobies. You'd have a variety of temperaments as well, from the smart, territorial chromides at the bottom to the active platies in midwater and the aggressive, fighting halfbeaks at the top. Apart for the chromides, where a pair would be ideal, the other fish would be good in groups of half a dozen or more.
Cheers,
Neale
