0.6mm on thin stuff for me. I have used 0.8mm but don't feel it's quite as controlled if that makes sense.
I have a 25 year old Sealey 150A mig so I can't really comment on whether modern mugs are 0.6 capable.
so is gas brazing , but if you compare it to tig its not too bad , the nice thing is that oxy acet kit is practically free these days , and once you have the gas set up it has many many uses.