I don't know the answers to your CO2 questions. I don't use it. I just wanted to say that you don't HAVE to add CO2 to have a planted tank. I (and many other people) don't. What you HAVE to have are:
a good substrate (assuming you want to grow things that grow in the substrate)
CO2 is necessary once you start putting lots of light in your tank (over 2.5WPG) and will make your plants grow faster even if you have less light, but if you want to start cheap/simple you could limit yourself to 2 to 2.5WPG of lighting and not bother with the CO2 at first.
Regarding your specific questions, Simpte will be along soon with some real answers!