Icedcoldmine
has 3 males in a 75 with no problems. If there are a few females and plants to hide in, I think it should be for sure fine.
Good tankmates for bettas could be just about any rainbow/blue eyed honey, a lot of the smaller tetras, many small plecos, some other gentle fish that won't look like other bettas, but in a 55, you might even be able to get away with dwarf gouramiis and guppies, as there may be enough room for everyone to have their own space. If you have a big enough tank, you can keep just about any two fish together, and when the fish are small, the tanks are magically bigger.