No angels, no other cichlids, maybe some bottom feeders, platys/livebearers, Ember tetras, Harlequin Rasboras, but not many overly colorful fish, as those fish are generally seen as reg flags to a betta and can instigate some hostility! The unofficial, although relative to the situation and successful THEORY is, the larger the tank with a betta and their tank mates is, the more room they all have--thus, it diminishes the chance of any malicious intent towards all of the other fish. For example, with a decent sized school of tetras, with any of the betta's possibly pending negative temperment, it cannot as easily single one fish out of the school out to harass and attack. Although if there were let's say 1-2 tetras, it'd be much easier pickins! I've slowly acclimated each betta into a mixed/community tank, and it works every time, And also just give it time as well. They ALL ended up getting along with each other. Although 1 tank does have a 1 Krib, 1 betta, 10 Lemon tetras, 8 platys, and 4 snow white guppies. So technically the krib IS a chiclid, but much smaller than the big King betta lol. He's the dominant, alpha of the tank. : ) And instead of fighting, all they seem to care about is food anyway! lol...all of my bettas have been successfully integrated with nobody getting hurt. Just my suggestions on which fish can end up being decent tank mates. Or maybe I just get lucky!
Most people set up some cories with them, I can't remember the types that are temperature compatible though, sorry. Since it's a sorority, I wouldn't recommend mid-level swimmers like most schooling fish, especially fish like ember tetras or harlequin rasboras as they're too flashy and brightly colored for most bettas: they become targets. I'd just stick with the cories and bettas, and good luck!
I think 5 females bettas with a school of 8 or so kuhli loaches would do well. As the one before me said, longer tanks are better but we can make it work.
Another idea is pygmy corydoras, you could easily do 8 of those instead of the loaches. I personally wouldn't bother adding any of the full-sized species of corries, a normal 20gal doesn't have much more foot space than a 10gal, which is on the small side for most corries.