I have kept sunfish quite successfully in a 30 gallon.
They do get quite territorial, so it is a challenge to find the right mix. Obviously, the smaller, the better. The advantage is that they are quite hardy, and just as interesting as many tropical species. (Not to mention, cheaper!) Currently I have two green sunfish about 3 inches long, and a bluegill about 4 inches long. If the largest fish in the tank is peaceful, it will work out beautifully. However, if he is agressive he will terrorize the other fish in the tank until they get very stressed. Add the fish at the same time so territories can be established from the beginning. Adding new fish could be a disaster, and should only be done if it is bigger than the other fish in the tank.
These fish eat almost anything you throw at them, but are very messy. If you do frequent water changes and have sufficient filtration, you shouldn't have any trouble.