My thought is anything under a 12 gauge for geese is just plain irresponsible hunting. Turkey on the other hand, the shot is relatively easy althought the target is extremely tough, someone who has practiced alot with their shotgun maybe a 20 gauge would be fine. But waterfowl with those big heavy birds blasting by you if you use anything under a 12 I think you are doing the game and name of hunting a great disservice.
All the best