Is this the aquarium?
biUbe Aquarium Kits with Light at PETCO
For something like that I would suggest 6 or so small fish like rasboras or smaller tetras. They should all be the same type. Harliquin rasboras, lemon tetras, flame tetras or some thing similar would work.
Then I would add about 4 small corys, they will stay near the bottom most of the time. Some get larger than others, but I know that the cories that are often sold as julii (they are really trilineatus) stay small.