I am getting a flame or a powder blue gourami, or a school of fish. By the way i have a 12 gallon tank and would one otto catfish be any useful to cleaning the algea or will it just be a waste of space.
Dwarf gouramis or honey gouramis would be an appropriate choice, perhaps a pair of them, as for an otto catfish, they are very sensitive, perhaps a better choice would be a few pitbull plecostomus.
If you do want a shoal of fish, jellybean tetras stay very small, you can keep quite a few in an aquarium of that size.
As your tank is rather small, a school of fish would be hard to do, as most schooling fish grow up to 2 inches and you really need 5-8 fish for a good school. You could try neon's, cardinals or any other really small school in about 5 fish to see. Gouramis, especially blues, get bigger and would be better in a bigger tank. You could do a pair of dwarfs, though. They are very pretty little fish and then get two cories so you would have fish at the top and bottom of the tank.