Mollies and platies should go fine together! As thay are both tropicals and live bearers.

And about putting them together with goldfish, the platies would be better since they can handle colder temperatures perfectly, which the goldfish prefer. But then again Mollies don't mind a 24 degree water, and I've seen myself that mollies go very well with fantails (: but do be careful with buying too small specimens as the goldfish would probably just have them for a snack!
So there is a rule that since goldfish prefer their water cold and alkaline, most tropicals would do better living on their own in a heated tank.