My goose was raised along with the ducks, so they've never been anything but family. I sometimes rearrange my flocks, and I've not had any trouble with integrating new ducks either. I don't know whether it's important to raise the goose with ducks so they identify with the species or not, but that's what I did. When introducing new ducks, I use the same precautions I would if I didn't have a goose, and the intros have always gone very well. Once the goose asserts his dominance and the duck accepts, they get along just fine--he doesn't try to hurt them.
Now, on the other hand, he wants to kill the ducklings BAD. As soon as he saw them in their outdoor enclosure he started hollering and came flying--flying, really--down from the patio where they'd been foraging, and began trying everything he could to get at them, hissing and honking and basically being furious that the little buggers dared to infringe on his territory. So I would not ever let ducklings within his reach. But full-grown ducks, no problem.