Aluminum MIG welding uses a special gun. The spool and feed are on the top of the gun itself. Here's a picture of an aluminum MIG setup. You can the gun in the pic, it uses 1lb spools on the top of the gun
This machine is an ESAB dual process. It has two guns, one for steel and one for alum.
I've seen ppl convert standard steel MIGs to run alum, but it doesn't work well. The alum wire is very soft, so it kinks and jams up easily. You also have to completely clean out the entire machine when switching back and forth. The lining of the steel wire will contaminate the alum weld, and vise versa.
As for TIG welding aluminum and SS. Aluminum requires AC and I've never back purged it, and I've always been fine.
Stainless needs DC negative electrode, and it does need to be back purged. If you don't the backside of the weld gets contaminated and will become weak and crack. The weld will start to turn white and will be brittle.
Back purging is always necessary when enclosing a tube entirely. If you've ever completely sealed something w/o back purging, the air inside will blow out the weld at the very end (usually just a pin-sized hole)
IMO, for fabbing up intakes, exhaust, turbo plumbing, etc. TIG is the only way to go. Also with all TIG process's 100% pure Argon is the only way to go. I've tried to use steel mix to TIG steel, it works, but argon works much much better.