Try 2.5% CO2 in Argon - it will MIG weld 3-6mm no problem and will also MIG weld stainless without any issues.
Any mixture with more than 3% CO2 will affect the chromium content of the stainless, meaning the weld will rust. Below 3% there is no effect.
MIG/MAG welding should be around 15l/min - position of the torch will have no effect on the gas, but will have an effect on the weldpool - i.e overhead you could end up wearing more weld than you put down!
TIG welding flow rates are 4 - 8l/min
Helium mixtures need ahigher flowrate as...