Some bettas have a problem with snails and will rip parts of snails bodies (eyes antenani) thinking they are invading their territory.
If you have a male, shrimp are also not the best choice. My betta Neptune ate 3 ghost shrimp of mine.
I would recommend Cory Cats. Fish with bright colors, and long fins are not reccommended as your betta may kill your other fish. There are many other threads about this topic, I would recommend searching around some of them and reading others pros/cons about certain tank mates and go from there.
Boys: Rosewell and Ren
Girls: Juno, Circe, Iris, Rhea, Calypso