My ducks just know to stay out of the way of the geese. My geese are having epic battles with each other, because, you know, ganders and spring, but there is no fighting between the species. My muscovy drake did a few rounds with the ganders, and emerged victorious, so although the ducks move out of the way, the drake still reigns supreme out in the yard. I have kept geese in my aviary with my pheasants and teeny tiny quail as well. No fights there either. It may depend on the temperament of the individual geese.