I personally wouldn't put either mollies OR platys in with a betta.
I have both dalmation mollies (lyretail) and platys, keep them in a 55 gallon heavily planted community tank.
The mollies can be pretty nippy, so other fish need lots of hiding places.
Also you need to be particular about the sexes of fish you attain. They are live bearers and the males will keep the females pregnant and basically wear them out, so you need more females than males, very important.
Cool fish though, my mollies are entertaining.