Rainbows are rarely aggressive toward other fish, with the exception of competing males, which is really more a game of chase and showing off colors more than anything. They are one of my favorites, with about 12 (boesmani, turquoise, austrailan, red irrian, praecox etc.) in my 125g planted community now.
One thing to keep in mind is that they are very active and fast swimmers, which can drive some of their tank mates nuts. One or two smaller ones in your 20g may be ok, but keep an eye on your platys for signs of stress.