I've kept loads of Bettas with quite a variety of fish.
Typically avoid cichlids, other labyrinth fish, the larger barbs (Tigers are horrendous), larger Tetras.
Most small species catfish i find agreeable, along with Loaches. Neon, Rummynose, Cardinal Tetras all done well. Lemon and Silver Tip Tetras ive had hit and miss, atm i have a breeding group of Silver Tips so they pay no attention to anything.
Golden, Cherry, Black Ruby, Rosy and Cheque barbs ive not had issues with, and ive kept Crowntails abduction Halfmoons with them.
I have kept Leopard Spotted Danios with Bettas without problems, Zebra and Giant Danios have been buggers though.
Sharks ive also kept. Redtail, Red Finned/Rainbow, Black and Bala/Silver sharks all quite happily kept with them.
And then Live bearers. Mollies, Platies, Swordtails and Hybrid guppies (captive bred wild x fancy). Endlers and pure fancy guppies usually become snack foods.
Obviously, some of thsee will be too large for your tank, but some ideas for you
