My male Betta attacked my neons t. They were large so I figured he couldn't eat them. I had one in the past eat smaller ones. I've seen comet gold fish try to eat the fins off a blue betta. mine does well with platys, rubber lip, and loaches.
Be careful I have had a betta that killed 3 other fish ! The betta is also slow swimming so they usally get their fins nipped by the other fish! Peaceful things like ghost shrimp,cory's,neons would be good too, nothing that looks like a betta like fancy guppies or anything that nips fins or the betta can get shredded!