Platy species are almost becoming indangered. There are so many people breeding this and that and crossbreeding the species that the pure species and colors are going to go the way of the dodo if we are not careful. It is never a good idea to crossbreed. It is like trying to breed in a defect. It makes just one more wrinkle in the fabric of life and can cause a problem in dealing with fish constitution at a later date. It usually does not end up with the best of both species, but the weaknesses that show up. I would definitely think twice before doing anything of the sort.
Please consider what is really best for the fish, and not what would simply be possible to do.
As far as how many fish you can have in a 20 gallon, it truly depends on what fish and what their temperment is. If you chose the fish that are peaceable and small you can have the rule of probably 1" per gallon without tail fins. (so 20" of fish grown-up size not the size when you buy them) If you buy fish that are territorial or semi-aggressive or aggressive the number of fish goes down as the rest of the fish will not have access to their part of the tank. You also have to keep in mind that you need to make sure not to overload any one level of the tank. Not all surface dwellers, or middle or lower. When you have a list of possible fish, why not post them and then we can pehaps give a better answer.