I think you could keep the platies in the 15 gallon with the cories if you wanted to keep them. If not, you could look at tetras (pretty well any type). There are many options for that 15 gal, but make sure you don't get fish that are too aggressive for the cories (one of mine came from the store with only 1 eye due to a tiger barb attack). Have fun with this though, there are many good websites that you can get quality information from, and I find that it is SO important to do your own research because the stores don't usually know anything!