All the loaches are going to get too big (well... botia sidthimunki would be ok if you can find them, but they're a bit pricey to risk putting in w/ a betta). A Kuhli loach would get too big, and it wouldn't be too happy w/o other Kuhlis anyway.
A Cory would be an ok companion. Your Betta will likely leave it alone, and it won't get too big. You could probably get 2 dwarf corys (or 3 if you're diligent with your water changes). Corys prefer the company of other corys, so dwarfs would probably be the way to go.
An Otto (2 if you keep up w/ water changes) would be a pretty good companion, as well.
Someone mentioned shrimp, and I've found they do alright with bettas as well. Your betta will undoubtedly nip at them, but as long as there are places to hide your betta will most likely never catch them.