I wouldn't put mollies in a tank that small. They even have a hard time in a ten gallon. They can grow to be 2 to 3 inches each and will need more water. I had a molly in a ten gallon with platies and it was not happy. I traded it back to the store. :/
Besides what the others have said, Mollies are actually a brackish to marine water fish. They will live in fresh water, yes, but do so much better in brackish water, and i have even seen them in salt water tanks. Not only that, but they are a schooling fish, and need at least six in their schoal.
I had mollies and bettas together a couple of years ago and they did fine but now that I have started fish keeping again my new bettas are just too aggressive to be with other fish. One of them actually killed my male molly because he thought he was challenging him. And I am told that mollies don't like to be mixed with other fish because it stresses them out too much. They prefer to be in their own species tank and I've been finding that they are happier now that they are by themselves rather than mixed with my guppies and betta. You always learn something new when it comes to animals. It's amazing how much we actually don't know and have to learn through trial and error.