My American Buff geese coexist with my Welsh Harlequin ducks quite well - even when they are sitting on eggs or raising goslings. Basically both species ignore each other. The geese won't protect the ducks. The ducks do give way to the geese as necessary, but the geese are pretty laid back and fairly tolerant, as long as the ducks give them space. They do have a large area with lots of fresh grass, and plenty of water sources.
My first batch of geese was raised with my first Welsh Harlequin ducks, and one of the drakes would try to breed with the female geese. He would chase them all over, and the ganders would do nothing till the goose was close to them, and then the gander would hiss. The drake backed off . I would have preferred the ganders to be a little more forceful then!