+1Lambchop rasbora, Trigonostigma espei, and the good old standby white cloud mountain minnow. Or any of the other Tanichthys species, for that matter, as well. T. micagemma and T. thacbaensis show up once in a while if you don't want to do the more familiar white clouds. All these are large enough to not become betta bait but small enough you could do a reasonable school in a 10 gallon with a betta. They also won't nip at the betta's fins like many danios would.
WYite