1) Under barrel first. More in line and lower to your shoulder which helps you recover from recoil a bit better. On O/U's without a selective trigger, the UNDER barrel always fires first.
2) IC/M over a pointing dog. Early season I sometimes use SKT/MOD. When birds flush wild or out a ways, switch to MOD/FUL
3) 6's. You'll get less cripples. I prefer #5's later in the season. Over a pointing dog I get away with 7/8 oz. loads. Will switch later in the season to one-ounce loads.
4) A lot of vests and coats out there to choose from. One's with many cavernous pockets to carry lots of stuff are best. You carry more than just shells afield threse days. I.E. Water, first-aid kit, compass, etc. It's nice to have something to carry them in comfortably.