Check out nano tanks on the web- that should give you an idea. My brother has a 6 gallon nano tank with a pc lighting system. He has in it live rock, a deep sand bed, numerous mushrooms, a clown fish, a watchman goby, and a garden eel. After two months everything died, Nitrates and salinity was the cause. When dealing with such a small water volume, fluctuations in the system are more drastic in smaller tanks. In a larger tank you have more room for error. If i would to setup a nano tank I would mostly make it into in invert tank (you have lotsof options when it comes to inverts), and I wouldn't put more than 2 fishes in there - the more fish the more nitrates. You should try to do some research on nanotanks.
Good luck