Hi! I have two geese that were raised and brooded with ten ducklings (they adopted the ducklings as their flockmates) they are excellent guardians (they're loud, warchful and big) but can also get a little sassy. They are both female, and I know you mentioned above about the possibility of a male goose mounting a female duck, and although I haven't experienced that myself as I don't have any ganders, I witnessed my female goose mounting one of my layer ducks...not good at all, especially since it was out of water and the goose is so much bigger. My duck was a little shook up but I was able to break them up.
Also, as mentioned above, my geese get VERY worked up if the female ducks begin mounting one another in or out of water. They honk loudly and display aggressive out stretched necks until the ducks dismount and waddle away.
I noticed that the geese are very close and tolerant of the ducklings they were brooded with, but the older ducks tend to steer clear as the geese can get a bit aggressive with them.
There are certain breeds that tend to be more docile, I have an American Buff (very calm, chill) and a Roman (bossy, alpha, louder, aggressive) both are supposed to be rather subdued, but each bird will have their unique personality.