Personally, I've never had a betta that was too aggressive to live with something. All but one did fine even with very small fish. The exception got along well with small cichlids and livebearers as big or bigger than himself well enough. Both my current males are so mellow they don't even mind cherry shrimp.
I would recommend a small group of endler's livebearers (probably only males, prettier and smaller and no overpopulation possibilities), a shoal of pygmy corydoras catfish (3-6), a trio of guppies (iffy with their fancy tails but I've never had a problem with it), a trio of platies, a school of otocinclus catfish (3-6), or any of the freshwater shrimp - especially amanos and cherries. I've never had any luck with ghosts. Obviously not all of these together, but for example a tank with the betta, 5 cories, 3 endlers, two amano shrimp and 5 - 10 cherry shrimp wouldn't be overstocked but very interesting. I'd also recommend nerite snails for algae control (but not together with otos - not enough to go around). Very hardy and my favorite algae eaters.
Be warned, amano shrimp climb out of tanks like no tomorrow if frightened by a fish or not happy with water quality, and need a tight fitting cover. I recently bought three, and lost three, all in one evening because my betta terrified them. They don't just jump like other shrimp and become limp on land, they climb plants or wires and run away like cockroaches only to dry up in a back room somewhere you'll never find them. Cherries also bail when scared but not nearly as readilly and they won't climb.
If you don't like more red, they also have blue, white, yellow, and a few other breeds of cherry shrimp but they aren't as readilly available.