Do you know how many watts per gallon your lighting is going to provide? Im going to make a guess that if its a single flourescent strip you have it'll be an 18W bulb, which will be kinda low for a 20G. If you can fit another light strip on there you'll be right on track.
Ive got that headache just now, I have co2 and fertlizer systems set up but because of my low WPG due to a fixed fitting on the hood, I'm limited to what I can plant.
Also, you say you added some rocks you had lying around. You didnt say if they are from an old aquarium or not, but if they arent then it might be an idea to boil them. Also, you should be aware that some types of rock will alter your PH, and some will release potentially harmful metals.
You do seem to know what you are doing anyway. You can read and read and read, and listen to every opinion there is, but you'll learn more from your own experience, and mistakes in my case. Im no expert but you seem to know what you are talking about anyway! Cant wait to see the finished pictures!
Any ideas on what fish your gonna fling in?