I second the bottom-dwelling suggestion. If you want mid-swimmers, however, my top picks would be:
- ember tetras (soft water). They are really pretty, especially in a planted tank. They aren't so bright as to infuritate a betta and are very peaceful. They won't nip and swim slowly so they won't stress a betta out with fast movements (although they will high-tail it out of the way if chased).
- Endlers Livebearers (hard water). Also incredibly pretty - a good alternative to guppies. I'd suggest 4 males. They are very small, so not likely to irritate bettas, and the males aren't nippy. :) They also won't overcrowd him as they only need groups of 3, unlike schooling fish which prefer groups of 6+.