I have had plants in gravel and they did fine, right now I am trying sand over potting soil and I have to say that my plants are doing great. I have never tried any really touchy plants.
plants in gravel you can vac around them, stirring it up like you normally would, with sand I am learning to hover just over the sand
I wouldn't worry about co2 injection in a 5 gal tank
Some people say that you need 1.5 WPG (Watts per gallon) others say 2+ it also depends on the plants you want to grow.
There are some really good fact sheets
here that can give you some good information.
Listen to Sue, she has much more experience than I do, and she snuck in on me,lol