I think you could get 6-8 pygmy corydoras catfish. They're tiny little guys (about an inch long max) so would have a very light bioload. They also stay on or near the bottom of your tank, unlike your betta, so they would stay out of each other's hair. If you do decide to get them, I would only add three at a time a week or two apart to avoid going through a mini-cycle, and they would really benefit from having a sand substrate to sift through looking for food.
Of course, every betta is different and some are far more aggressive than others, so you should still take care to be sure that the betta doesn't attack them.
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