Like a few have already said I have no issues housing ducks with chickens and even guineas right now. It is normally because something happens to the other ducks we had. I had a pekin that I house with my flock a few years ago and now we have a rouen with our current flock. Until later today or tomorrow I am getting a few more ducks. Only thing is that i did like many people said brooded them seperate. And the ducks I found you might have to seperate by breed if you have light and heavy. My pekins kinda smashed my rouens when they were being brooded on my 1st go around with ducks.
Right now our rouen calmly follows the guineas or chickens around staying back a bit, kinda like the lone tag along. We did have issues last year with a rooster trying to breed her, guess the drake x hen is same for Roo x duck. Our roosters are kept in check mosty by the guineas unless its heavy mating season then the birds kinda each go their own way. The duck did learn however she can squat down and make it so the roos can't mount her.
My birds never seemed to bother each other with the exception of the roo/duck dealing. The hens just don't seem interested that the other one looks/sounds different. I don't think she is "IN" the pecking order.
She does make a lot more noise than the hens, but not as much as the roosters or guineas, but normally when there is something that startles or worries her.