I had a few small platies in with my betta. He and the male (a peppermint) didn't see eye to eye, but everyone else was fine, including my ghost shrimp, baby frogs, and my snails (rams, ponds, assassin, trapdoor).
Mike had a female betta with his guppies when he first started, she killed a bronze cory, and ate a couple of the adult females. She also killed some shrimp, and has eaten snails. She HAS to be kept by herself.
The point of those two stories were that some bettas will live with other livestock, and others will destroy anything you put in with them.