African dwarf frogs, snails, and shrimp are the fish/others I can think of on the top of my head that would go amazingly with bettas in an 8 gallon tank.
There are other options out there, too. However, those are probably the best.
To be compatible with a betta, the fish must...
1. Doesn't look like the betta (no flashy colors)
2. Live in the same water conditions
3. The right size for a tank
4. Not a fin nipper
Good luck and happy fishkeeping!