you can put a valve on the return side of things and trim the impeller a little bit I dont think it would be a good idea to trim off to much. the light might give enough heat but a heater wouldnt be a bad idea just incase. you can try
http://www.nano-reef.com they can help you out there. I had a 3 galon this summer I had a small goby, small perk, 3 hermits and a turbo snail. you can also do a sexy shrimp.
hpe this helps
Brad