a snail? corys wil be fine with your betta, but not sure if you'd have enough space even for pygmy corys. maybe shrimp?
your betta will be happy alone, so it's really for your enjoyment or as a cleanup crew that you'd be getting other fish imo.
there's probably some good suggestions for betta tankmates in the betta forum, or you could ask some of the betta lovers here what they have....actually i just thought about that, that was probably the point of your thread....sorry.