SC was invented a long time ago in the UK as training and practice for hunting. It's the only shotgun sport I'm interested in.
I'd stick to the 12 gage. Adding an adult buttstock is usually easy. Going to a 20 gage is often not the way because the 20s are usually lighter so the recoil is the same as the 12.Women for example just shoot 12s as I've heard.
If your interest is also in hunting just pick a good hunting gun as I have and you're on your way. A used gun in good condition is a good solution .As they get older they can make up their minds as to what gun they want . I have a double and an auto .A pump always takes time to get the second shot off so I wouldn't suggest them.
And Watson , bring your revolver !