okay markers need a certain amount of input pressure to work! i think co2 gives out a very inconsistant 800 psi input pressure in to a marker and this pressure drops dramatical under rapid firing of the marker causing your marker to draw liquid and also a drop in velocity a melee is all so set to about 800-850 but doesn't really suffer from any drop off as it uses air
sorry this is a really crap explanation but a melee would work great on a mag
You need a srew in tank, pmi are pretty good ones. If you want to spend money there are some very consistent regs out there that arent screw. you can use them too but they are harder to setup then a screw in.