As long as the tubing is cleaned any cup/set up would work. That being said, I would use a #5 cup assuming your going to be in tight spaces. Ceriated tungsten should be orange or gray I believe. For a miller diversion I would just grab some 3/32 2% red thoriated tungsten. Hopefully your using 100% Argon as well. I would also use either 1/16 or 3/32 filler , depending on how tight your fit is. Remember, your only trying to make 1/8" welds. Not much bigger if you can help it.