A 10 gallon is a rather small tank. I would not recommend you keep anything larger than a pair of smaller gouramis, maybe a pair of dwarf gouramis. And who knows, they might even breed. This could make the tank even more interesting to observe provided you can find a place to keep the fry. Small non-aggressive cichlids such as rams would work, as stated above. But if you choose to keep cichlids, I would strongly advise that you stick to the least aggressive species as 10 gallons is hardly enough room for the fish to establish their own territories.