They refer to the diameter of the fluorescent tube. T8 are fatter around than T5. It is likely a hood that came with an aquarium is T8, but without more information there is no way to tell.
You can use sand or planting medium substrate like "Eco-complete" (one of many brands). Gravel is not generally recommended.
No, you do not always need CO2
for plants. Many of us do use a liquid that provides CO2
that you will see referred to as Excel, CO2
Booster, or glut. Some use nothing to provide extra CO2
Most of us use some kind of fertilizer for our plants. This includes liquid that goes in the water and root tabs that go in the substrate.