Adad, #6 shot oughta work fine. If you get the .410, have a gunsmith open the choke to modified or improved cylinder. Because of the small shot charge, most .410s come in full choke, which shoots a bit tight on the pattern.
If you're hunting in large-tract areas, .22 flyers aren't that much of a problem because most of the misses hit wood. Again, ya gotta pick your shot, not just let fly at a squirrel sitting on top of a limb with no vegetation behind him.
The two-person deal is that one hunter walks ahead and the squirrel circles the tree and hides from him. The hunter some 20 or so yards to the rear gets the shot. The shotgun works best on those squirrels going from tree to tree in a hurry...