My betta gets along great with my otocinclus catfish. They do a great job with algae, are active in the bottom and mid-levels, are very peaceful, and not flashy or threatening to him. He'll even go hang out with them, and when i give them a zucchini treat they all share it with no problems. If you go with the otos, make sure you get at least 3 as they are schooling fish, and because they are so peaceful having a group of them will keep them healthier, happier, and free of stress. Plus, they stay right around 2", so you don't have to worry so much about them outgrowing the tank like a plecostomus would. They are a lot of fun to watch, too!
Pixel art thanks to MameJenny