I'm not a fan of the 20 gauge with buckshot. The only readily available load is #3 buck, which is a bit light and may not penetrate enough. It also tends to pattern horribly.
Most folks wind up going to 3" 20 gauge shells to get a decent payload. They recoil as much or more than low recoil 12 ga rounds. I'd stick with 12 ga and low recoil, and a shortened stock. Even small people can handle these just fine.