1)Snows and Blues are toughest.
2) small Canada geese
4) specks - easiest. they fly early in the morning, in small flocks, at low elevation.
Snows: Impossible unless your in the hot field, under a big flyway, and the its that one in every 5th year. Have had three flocks of 10 - 20 young birds come in right behind each other (not one adult in any flock). The birds would decoy the falling shot birds in the flock ahead of them. Passed on the third flock and let them land.
Squeaker/small Canada Geese are great pass shooting geese. The establish tight morning flyways and typically fly at 60 yards or less until someone pops a round or two at them.
In the decoys : If the first flock or two of small Canada geese flare at 60 yards out or so. Get out of the decoys, hide in the field, blind, or a rock pile anywhere about 50 - 75 yards from your spread. Pass shoot the geese as they are dropping in. Of course that is when the mallards sit right on top of the spread.