In how big a tank. With what type of ornaments and gravel?
If it doesn't have a filter, then he might have trouble staying alive, and the food you toss in will rot b/f he can finish it. However, tubifex worms stick to the sides of tanks and can be eaten by the betta and the oto. I suggest asking more questions b/f buying the oto (not the betta, you're fine w/ what you have for now, though a book might help) along with the fact that you'll have to cycle the tank w/ the betta in it b/f you can add the oto.
Algae shouldn't be a problem if that's why you want him, though.