I think the type of fish you are referring to is a Mickey Mouse Platy.
Bettas 'can' be peaceful, but I wouldn't suggest mixing them with Platies. I know some who would have a betta do very well in a community tank for a few months and then the Betta would go nuts and attack. It really depends on the personality of the fish, but their other name is Siamese Fighting fish.
Some fish that usually are good tank mates for bettas are corydoras and neon tetras. Cories need to be in a group of 5 or more and the tetras need to be in a school of 6 or more.
I say cycle your tank first (link below please read up on that) and just have a betta or have only other fish. I don't think they will work out together.