You really should keep them in the set up tank, as the water conditions are right. Get a breeder basket if you can or a small container that you can punch holes in the bottom and hang into the water on the side of the tank. They can store sperm for 2 years, so you might have babies long after you buy them. To feed the babies, crush up flake food as fine as you can, or take the yolk from a hard boiled egg and dissolve it in a cup of water then feed the babies a few drops of this water twice a day. What do you have in the tank? Any livebearer will eat their own babies.

Also, platies and mollies are in the same family, so they can interbreed.