The guppies and platys will hybridize. Moving adult livebearers or the babies after they are born will increase the number of fry that make to adulthood. Adding dense cover like java moss to the tank with the parents helps as well. There are plenty of tank mates that you could choose.

20 gallon and 10 gallon guppy breeding.

I am currently cycling and setting up a 20gallon tank, I have a 10 gallon currently with 1 male guppy, and 4 females and 2 male platies. I want the guppies to breed which they probably will. Should I move the parents to the 20 and let them continue to breed and move the fry over to the 10? If I decide to do something else with the 10, are there any tankmates these guppies can have in the 20 gallon that WONT Immediately eat all of the guppy fry?