Howdy Wandering Man
Welcome to CAS, you and your Wife are in for a lot of fun, you ask about shot shells and shotguns, The 12 gage is used by females too, but if you like the 20 gage thats ok too, everything is a matter of what you like the best, or what is best for you.
I shoot the 12 gage and I use 7 1/2 shot you can use 7 1/2 or lower in CAS , and they also sell a reduced load too.
I have two guns that I use , one is a Winchester model 97 clone from IAC. and the other is a TTN side by side, both in 12 gage. Where I live they seem to have better deals on the 12 gage shell than they do anything else, so you might want to consider that , I load cartridge ammo but not shot shells, right now and for a while now, the shot shells have been pretty cheap, to a point, that it wasn't worth it to me, to load my own.
So this is just my quarters worth, I'm sure there will be others with different ideas on this, also to if you like lever actions, you might want to look at the 1886 Clone their a nice gun also.
Hope I was of some help on this
tEN wOLVES