I am trying to fathom the point at which you can start to use 1.0mm wire with a mig set. I am looking at sets in the 180/200 amp range and some of them say 1.0mm wire and others only 0.8mm. Anyone?
I can start you off with a guess, though it's only thinking about what I would do rather than anything official:
0.6mm to 2mm steel 0.6mm wire
1.0mm to 6mm steel 0.8mm wire
2mm to 10mm steel 1.0mm wire
You can get away with using thicknesses of wire outside the range - I'm sure there are folk that use 1mm for really thin stuff, and I've used 0.8mm on 8mm steel, though it would be more hassle.
Amps are very relevant - the wire controls them to an extent. You'd get away with doing everything on 0.8mm wire, but you might not get maximum amps without 1.0mm or maybe even 1.2mm at 200 amps.
The thicker wire conducts better so you get more amps and will be able to weld slightly thicker steel. It's the same argument for using thin wire on thin stuff where you want to reduce the current. Makes a real difference.