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+.