... #6's i would use for ducks....
I'd agree with what you said except when hunting duck which for most people is gonna be steel shot. Then I'd go to a #4 shot for puddlers in decoys and flooded timber birds. For larger birds at lake distances I'd usually use a #2 shot to carry a little more energy for the distance needed. My favorite, but which is harder to find is #3 shot because it's a really good working compromise between the #2 and #4 sizes. I always used T shot for goose, but admittedly, I've not hunted a lot of goose so I'm kinda hesitant to recommend a shot size for them. Moral of the story, if using steel shot go a couple sizes up for a similar effect as the normally used lead shot sizes.