We have a Welsh Harlequin drake and his two duck hens in with our chickens. He's fine with the chickens, and a whole lot quieter than a rooster.
Wetness is an issue, though. Ducks don't have to have ponds but I find they can splash water out of waterers and make muddy messes quickly. They can empty a chicken waterer in an hour, and they need their water at least 3 inches deep so they can clean their eyes and noses, so you can't just use a nipple system and keep them healthy. With a "pond" they never run out of water.
I am debating whether it is safe to put in sand to help make the yard they're in drier or if that would just be a breeding ground for bacteria, etc. or cause problems with digestion or crop impaction?
Previously we had the ducks separate from the chickens (except the momma Silkie hen that hatched them from eggs was with them). That coop had a rubber mat, and just enough slope that the duck "pond" - a 20 gallon cement mixing tub from Home Depot filled half full with water - could be dumped daily and filled fresh. Dumping it out washed most of the poo off the rubber mat, the ducks tended to poo mostly next to their pond, and any poo that was missed could be hosed off easily making clean-up really quick and easy. The ducks couldn't dabble mud into the water due to the rubber so it stayed cleaner than otherwise. The lawn looked FANTASTIC where the washed poo drained to and there was no muck in the yard with the ducks attracting bugs and disease etc. Unfortunately I didn't figure out how to predator proof that space and we lost all the ducks and the hen that hatched them to coyote... So now the ducks are stuck sharing predator proof quarters with the chickens, but it's flat, and the duck pond end is becoming a soggy mess. Last Fall I dumped 10 contractor bags of raked leaves in there over top of corn stalks, so it took a few months to compost and finally get yucky/ wet but I need to do something to dry it up... and I don't want to take away the duck's "pond" in the heat of summer...