Hummm maybe a pic or two of what you have could help us help you faster? I mig everyday on auto body work and the co2 argon mix works well but tends to be a bit more expensive than straight co2. I use straight co2 because I am going to be cleaning and grinding off any spatter caused by the Co2. Plus Co2 is a liquid. When welding you use the head pressure gas off the bottle which means you get about 3 to 4 times the product out of a co2 bottle than a argon co2 mix at about half the price!
Typically flow meters are used as a delivery valve. It screws directly to the argon/ co2 bottle with a #580 CGA threaded nut built on the flow meter. The flow meter typically has 2 gauges, one reads the gas delivery pressure and the other is tank pressure. The delivery flow should be between 15 to 25 cu,ft. per hour. Co2 on the other hand usually uses a pressure regulator as an delivery valve. It looks just like the flow meter and mounts on the cylinder the same way. The fitting on the standard size bottle is a 320 CGS threaded nut. They make adapters so you can run co2 on a flow meter, but not the other way around. Your mix is about 2300 psig in the bottle when full. The co2 is 850 psig when full so the regulator cant handle the higher pressure of the mix.
If what you have is .030 Id us it. if your welding on a car made after about 1977 or so the metal got much thinner so it wont take the same heat as an older car. What you will want to do is tack welds! Just a small spot on the seam then move a few inches away and do it again. If its a patch do this skip spot weld process until everything is filled in and it looks like one continues weld. If it ever gets too hot to hold your hand on the body just a few inches from the weld it is getting too hot! Stop and let it cool before welding any more! Be sure to keep skipping around to minimize the heat build up on any one area. Turn the machine down as low as you can and start adding wire feed speed and voltage until you get a good tack weld without burning through. Make sure there is no wind or fans on that can blow away your shielding gas. Hummm that should get you going on this beautiful Saturday. Good Luck