I have a full 10 gallon fuge and i have two 20watt NO flourecents on it. Curious thing is I put some rock with xenia on it in there about 2 months ago and its still doing great!
You can go as low as you want, it just won't grow as fast. Personally, I'd put a 65w PC fixture or bulb over your 10g fuge. That's worked great for me. Super fast growth.