I have an idea as to how you could cut the pad. Get a cookie cutter in the size that you want and just a regular propane torch. Heat the cookie cutter and it "should" make a clean cut.
If you use a coating cleaning will be much easier. Most of the time you can you your same car wash soap with a soft brush.
Pinnacle Black Label Diamond Wheel Coating
The gauge is not to determine what pad to use it's just to tell you approximately how much clearcoat you have to work with. Say you have a deep scratch but you have a low thickness reading. The last thing you want to do is attack that with a heavy compound and wool pad.
Nice thorough write up. The wheels look nice too! I wish all Alcoa SRT wheels had that finish. I should have bought the Jeep instead. Year round drivability if needed, more traction and much easier to lift with a jack.