Just throwing out what I have personally done and seen. I have welded 3.5" carbon steel with a Mig and I have seen, 1.5" #9 Chrome Tig welded many times, pre heated of course. And I have seen schedule 10 stainless steel, and that's pretty thin, Tig welded. I even saw a welder Tig weld a dime to a stainless fitting once. So, thick or thin, it can be done if you have the experience. What does all this have to do with welding aircraft tubing, not a d*()^ thing.
Bob