In My Experience, dwarf sag will grow just as fast as any other plant in my tank (Im sure they would be slow growers in low-light tanks, but not highlight). I have some in the corner of my 30 gallon and its start to grow pretty big, sending up runners everywhere.
You have many choices when it comes to stocking your tank, such as: Neon tetra, Cardinal tetra, Cherry barbs, Celestial Pearl Danios, Sparkling gourami, Ember tetra, Pygmy Corydoras, Pristella tetra, Killifish (Smaller kind), Platy, Guppies, Betta (Choose tank-mates carefully), Lemon tetra, Endlers livebearers, etc.
I don't think you could keep a constant supply with Diy C02. I have something similar to diy C02, uses the same method, the Nutrafin Natural Plant System (With C02), and I have to change the mix every 2-3 months.