I shoot 12 ga 2 3/4" steel 6s at teal and early season bonus ducks. They are 1 1/16th oz I think. I wouldn't go below that in shot, but then again I don't shoot a 20. I think you could be very effective with 5s or 4s as well and I've used them, particularly when you are starting to see big ducks but still mostly teal.
I wouldn't worry about patterning. That concept came over from rifle hunting where there is debatably some difference among individual guns of the same make. With shotguns you either shoot it well and it goes where you aim, or it doesn't. 6s will pattern more densely than 5s. 5s more than 4s. 1 1/16th oz will be more dense than 1oz. I think you see where I'm going. It's not going to hurt you, but your time would be better spent shooting skeet or sporting clays and making sure you can hit what you are aiming at.
I've shot every make of shell ever produced. I once had a bad box of remingtons and I've avoided them more since then, but I still shoot them occasionally and I've never had another problem. I've also had brass expansion problems with the cheap federals and I've completely quit shooting them. I do shoot the good federals regularly however.
If you are going to be getting into big ducks I would want 7/8 oz. Good luck, take some pictures and let us know how you do.