Based on the rule of thumb for 1 gallon per inch of fish I would say for a 5 gallon you can only have 3 guppies. However, this is not the minimum for guppies, and when it comes to bettas, I like to give them extra room for themselves. Bettas like to have their own space, so I would give them alone 4 gallons of water; that's 2 gallons per inch of fish.
Point is, if you put guppies OR mollies in a 5 gallon tank, NO ONE is going to be happy. You can't even put cories in a 5 gallon tank and it being happy because cories are extremely social and active fish that like to be housed with at least 3 of their own kind with ALOT of swimming room.
In my opinion I don't think you should put anything with your betta in a 5 gallon tank. I think you should get a 10 gallon instead. Your betta will love the extra room.
I feel your urges though. That's why I have a betta in a 55 gallon tank with ALOT of other fish. I wish I could put other fish with my divided 10 gallon, but their own 3 gallons each is not enough room for other fish.