Do you have the wattage on the shop light?
Normally, the shop lights don't work that great for lighting. The shop lights spread light everywhere, not concentrating it all into the tank.
You might just be able to keep low low light plants.
The reason for the floaters doing good (I presume, anyways) is because they are so close to the light that it doesn't matter if the light is reflected in.
Unless you don't ahve a shop light at all, then I'd say ditch the idea of getting one, and upgrade your lighting accordingly (flouresent, or CFL)
As for your other questions, no you don't need special substrate for low light plants, even most medium-light plants, the only time you REALLY need it, is when you have a carpeting plant, or a very heavy root feeder at high light.
Co2 is about the same, if you keep it low to medium light, then you shouldn't need it.
