I would think a small group of 6 or so would be fine, as far as tank size. I've never had problems with embers being nippy, but he has short fins anyways. I don't know how well the betta would do as far as aggression, or if he might even be able to get one in his mouth, though. You have me on that experience. I know I wouldn't keep embers with most 4" cichlids, but bettas typically have smaller mouths (relatively).Was debating if I can had a small group of ember tetras with him.