I think it would be a mistake to top these. You have 10 plants and a tiny grow space--I don't think you are going to have nearly enough space, especially if you top these. That many plants in a 6 sq ft space will do far better grown as single cola plants, IMO. I have almost 20 sq ft and never ever put more than 10 plants in there and prefer 8 or less.
I also doubt that a CO2 unit for an aquarium is going to really have any effect on your grow. You are most likely better off just making sure you have good air exchange.
And as a side note, T5s actually put out slightly more lumens per watt than MH lighting (92 lumens per watt compared to 90 lumens per watt) and the heat difference is substantial. In addition, IMO, the T5 spreads the light more evenly and as mentioned, you can get the light right on top of the plants. You can purchase a 2' PPL T5 that has a 2' double tube that emits 5000 lumens each. You can buy these with 2 tubes (10000 lumens) or 4 tubes (20,000 lumens). I believe the 4 tube fixture is 2 x 2.