I have an 18 watt fixture over my 5.5 gallon. Well, it is a 2x18 watt fixture but I found that things got too intense when using 2x18 watts over the 5.5 gallon. 1 bulb over the 5.5 gallon grows everything fine for me, and I'm using pressurized CO2 as well. My blyxa japonica even gets those reddish-tints to it!
I'd say 1 18 watt bulb over a 5.5 gallon would be in the medium/medium-high light category. You should be able to grow pretty much anything with that light, provided the reflector is pretty good and you have a good source of carbon.