You probably didn't get hurt as long as they didn't charge much extra for it but I doubt it will help with the thin stuff very much. There are all kinds of gas mixes out there, some even have more than two. These tri mixes can even have small amounts of Oxygen which seems to be a contradiction but in certain conditions it can help, usually these exotic gases are for industrial use and the main concerns are speed/production costs more than anything else. Most of them will work just fine for the type of welding you are doing however and unless they charged you a big premium it should not matter much one way or the other. Actually depending on your welder, the wire you're using and surface condition you may very well see a slight improvement, or not, but I wouldn't expect it to be anything to get excited over.
75/25 has been the standard mix for many years and every ratio possible has been tried but the old standard C25 has always been the best for all-around use for a good reason,