" I did not know a spool gun was aluminum only."
Ron, actually the REVERSE is more true - as in, aluminum is (sorta, but not really) spool gun only -
1 - Aluminum tends to NOT feed very well when PUSHED thru a tube, altho if you're REALLY careful to keep the mig gun TOTALLY straight or nearly so, SOMETIMES it can work. It's just NOT worth the PITA to do so, so for MIG welding it's easier to
2 - Either use a spool gun for aluminum, or (B) if you do a LOT of aluminum welding, the larger machines (250 amp and up mainly) can use what's called a Push-Pull mig gun - a full size spool of aluminum wire goes inside the machine like steel wire does, and the machine's feed rollers PUSH the aluminum into one end of the push-pull torch - but that torch ALSO has a small drive unit built into the OUTPUT end of the torch that PULLS the wire- this is what nearly ALL "all day long" aluminum fab shops use; it's a lot more efficient to change spools every 20 lbs than every ONE lb...
3 - not sure about the small hobby class spool guns, but the 250 class and up ones will take either a 1 pound spool of aluminum wire or a 2 pound spool of hard wire (aka solid steel) - same size 4" spool, but with steel wire it holds twice the WEIGHT.
Back up and check the pics I posted a few posts ago; almost ALL the "aerial" welds were done with the 30A spool gun and my Miller MM252 MIG. Calm day, a couple CFH higher gas flow than indoors, works fine til the wind comes up - then you either quit, put up screens (wind block) or switch to flux core wire. The alternative is welds that look like you blobbed on JB weld with a broken spatula, or worse.
Each MIG process (mild steel, stainless, aluminum, etc) will ONLY work well with a specific shielding gas - for mild steel, usually either CO2 or C25 (which is 25% CO2, 75% Argon) This is for "short circuit" welding, other mixes are used for higher power/speed "Spray Transfer" mode.
Stainless can use a "Tri-mix" or, IIRC, a mostly Argon mix. Aluminum wants straight Argon.
For a REAL education, Jody's WeldingTipsAndTricks site mentioned earlier is what you want.
HTH... Steve