Most mech shims are now made of plastic (Delrin?) so, provided everything is clean, marking will not be a problem.
The largest size band is 34.9mm and the most common smaller sizes are 31.8mm and 28.6. Most LBSs will have a drawer full of the reducing shims. The difficult one is when you've bought a 31.8mm mech and need to shim it to 28.6mm because they're quite rare nowadays.