I keep 7 chickens and 2-20 muscovies together. Have done for two years now and do not find a problem, I feed them all the same (grain plus concentrate) they have shell and grit available.
The coop is part of the barn it's around 6m2 which is fine except for the couple of weeks just before duck slaughter then it is a little cramped. The ducks bed down in the straw right under the chicken perches, doesn't sound ideal does it, but apparently they have Teflon feathers! I do not keep any water in the coop, it is out in the run and then we have a baby bath outside that they use, they free-range 90% of the time. If I will not be home at dark to shut them in then I keep them in their run, which is around 50m2.
When we have ducklings I top up the straw every couple of days, just with a slice or two, to keep the top dry, without ducklings it just needs cleaning out a bit more often.
Last year when they couldn't go out for 8 months here I moved the ducks into an empty stable as they had to have indoors water then and they really do make a soggy mess of everything.
The pair of ducks I got original were used to chickens, but my chickens had never seen a duck. I had no issues with introducing them, and they never fight, When the ducks have small chicks he will stand between the chickens and the chicks and make it quite clear he won't tolerate any funny business, but not a single feather has flown.
My only warning is that a muscovy duck will get into the chickens nest boxes, even though it's not really big enough, so you may get some squashed eggs. I set up a box on the ground for her when I want her to sit, I do have to fish out chicken eggs every couple of days from there.