Welcome to the club
We got 4 of the 20 GA HRs in the family
I got a Nikon pro staff on my wifes gun looking at the rings my guess would be medium and the hammer has just enough clearance to work without a problem . you do need the extension on the side of the hammer to get hold of it, are guns all came with it.
I have a Leupold on mine
We also shoot ssts