"Tap" machine means a machine with individual discrete voltage settings. A, B, C, etc or High/Low or 1, 2, 3, etc.
As opposed to a machine with full voltage adjustability. The dial may say 1-10, but you can set it to 3.5 or 4.8 or 9.2 or whatever you want.
Use C25 gas. It usually has less spatter than 100% CO2, which also typically runs 'hotter' than C25.
A higher voltage is needed to get the arc to run with 100% CO2 compared to C25. In short-circuit transfer mode GMAW (as opposed to globular transfer mode or spray transfer mode, which are usually NOT what you want to use on thin sheet metal, not counting pulsed spray), the voltage is about 1-2 volts less with C25 compared to CO2 for the same wire feed speed (amperage). More volts and same amps/WFS = more energy in the weld. More energy means deeper penetration (all else being held equal).
You can use a copper backer to help minimize or reduce blowing through a thin spot. Homemade from some copper plumbing pieces, or Harbor Freight used to have a 'welding spoon', which was a relatively thick piece of copper with a handle. Clamp or hold it FIRMLY to the back side of the weld joint.
Thin sheet metal is not the easiest thing to weld. Thin sheet metal that has random thinner spots is even harder to do.
Practice, technique (a series of small weld dots that you eventually fill in completely, as opposed to running a continuous weld bead on the seam), more practice, maybe using a copper backer or copper welding spoon, more practice.
A machine that can hold a consistent arc setting is a must. A machine with 'enough' adjustability to let you get the 'right' settings is needed. Enough could be either well thought out individual voltage tap settings or fully adjustable voltage settings.
A machine with only High/Low and Fast/Slow settings would not make it easy to weld thin sheet metal.