3” #3’s over a modified choke. Mallards over decoys to pass shooting teals. Keep it consistent regardless of what you shoot and you’ll figure out leads/distance.
You'd want to be prepared for, the longest range shot you feel would be likely to happen.
You can always just load up with the beefier shot size, and then if a smaller species presents closer by.... you can do what I sometimes have to do like when shooting doves coming in to perch near my dekes, and that is try to aim more towards head/frt shoulder region specifically. So if you're rolling with Modified Choke, and the shot is a little too up-close-and-personal, it becomes more important to remember to try for headshot-ish placement to avoid too much shredding of the usable meat.
So if there's chance of larger harder to kill species, I'd say go for the bigger shot size just in case, no?
Not sure why guys thing a #4 is "too small" for geese. I shoot #4s all the time and see no issue with them. Most guys not killing geese isn't due to shot size it is due to shitty shooting plain and simple.
I like 4 shot for just ducks. Shoot 2 shot for geese and ducks. Throw in the BB when it is late season geese. Once I have shot through all my 4 shot, I will probably just buy 2 shot for everything.
When I do use a 12 gauge I shoot mostly 2¾" loads of 1⅛ ounce usually 2 or 3 shot at around 1400-1500fps. If shots aren't going to be beyond 40 yards 3s fill in the pattern.
It's #2 shot. Enough BBs for a decent pattern and has the penetration to kill reliably out to 50 yds at the velocities of modern steel (you can look this up).