Are these tools appropriate for a roofing project?
I have seen nail guns that accept both roofing and framing nails; I would look for one like that. I would generally recommend a coil nailer, which holds more nails, and usually accepts roofing and framing nails (but be sure to check it before you buy it).

As for air tanks, I would look for something on the small side (like 6 gal) for roofing, because I find that you have to keep moving the tank. You also have to think about carrying or hoisting it up and down. Lighter is easier.