I use a Millermatic 250 and added a 30a spoolmatic that I use just for Alum and SS. I use .35 wire for regular steel and I have to make several passes on 1/2 to 1 inch to get a good bond but it works great in the middle sizes. I would use smaller wire on exhaust pipes and body panels. With the 30a spoolmatic and .30 wire I can weld alum tubing that is .058 wall and have no problem with it holding and .30 wire seems to work on SS fine although I have not tried it over 3/8 inch thick.
I have an very old dinosaur miller that I am trying to learn to tig on, I should have taken up juggling!