A 20-ga shell can easily lodge itself in a 12-ga barrel. So, it's not a good idea to have 20-ga shells in your pocket when shooting 12-ga.
also, the shell length is the FIRED length - do NOT put 3" or longer shells in 2-1/2 or 2-3/4" chambers because the unloaded ammo fits (like the "gunsmith" at Gander did to tell me the chamber length)
DON'T overthink choke selection for anything
DON"T go mall ninja and add all types or gizmos and gadgets that add more weight to your HD gun
DO go and practice after getting lessons - remember - "Practice does NOT make perfect, PERFECT PRACTICE makes perfect
A shotgun is not a rifle (except for deer and turkey hunting) do not aim, point it like an extension of your arms and eyes - your success rate will be higher