We acquired a trio of muscovies a few weeks ago. I'm following up with my experience, in case anyone else has similar questions.
Space: There's adequate room for the additional 3 muscovies in the coop. When everyone is up on the shelf for the night, it's plenty spacious. When they're all down on the floor eager to get out the door in the morning, it looks more crowded. Especially if our toddler comes along to the coop and chases them around. This is probably not a problem.
Pop doors: They use the 12x12" pop doors without difficulty. They don't jump as well as chickens, though, so it's important to have good access up to the pop door from outside.
Droppings shelf: The 'scovies experimented with perching on the 2x4" roost bar, and seem to have concluded that sitting on the shelf is better. This isn't as messy as I expected, as long as I scrape frequently. While the chickens plop, the 'scovies squirt. I've now got unappealing streams of dried duck by-product down the wall behind the droppings shelf. Working on a good fix for that. Duck doo-doo does add moisture to the coop, but the bedding is still able to remain dry.
Nest boxes: 'Scovies can't jump, so they couldn't get into the nest boxes. In the first days with them, we collected a few duck eggs off the floor but that stopped pretty quickly. It turns out our chickens had figured out how to eat the duck eggs. I re-arranged things yesterday so that the ducks can climb up into the nest boxes, and this morning found a duck egg in there. That's been the biggest frustration thus far but I think we have it fixed. Can't comment on brooding yet, since we haven't tried it, aside from saying that the chickens definitely won't let a duck brood on the floor. I also wonder if the small boxes we have will be too crowded for the 'scovies to safely brood there.
Self-waterer nipples: I introduced the drake and one duck to these right away and all three have had no issue using them since. I also put a traditional chicken waterer and a kid's sled full of water outside, and they use these extensively for drinking, nose-cleaning, and bathing. I think we will want to keep something like this available year-round. The 'scovies drink a lot more water than the chickens.
Layer mash: The drake eats it, and the ducks might as well. Doesn't seem to have any ill effects, but then they have plenty else to eat and access to standing water.
Free choice feeding: The person we bought the ducks from warned us that they would eat us out of house and home if they had unlimited feed. I'm not yet sure if our feed use has increased disproportionately. We're feeding a lot of whole corn treats right now to win over the ducks so comparison probably isn't meaningful yet. Today's duck egg was the size of a big chicken egg, for what that's worth.
Closing time: I've been closing the coop after dark, and the ducks are consistently inside and up on the shelf. I think they do stay out maybe 30 minutes longer than the chickens.
Social integration: Initially the chickens oppressed the ducks. I think the ducks are much more gentle and peaceable than the chickens and would prefer to be left alone. But they are starting to assert themselves and take places in the peck order when the chickens become aggressive.
Ranging: The chickens were damaging our plantings, so now all the poultry are confined to a yard made of 160' of 48" Premier 1 Chicken Net which we rotate every 2 weeks or so. It's usually not energized, because the fencer was annoying me and we have little kids, but it contains them fine. We did have one duck fly out but it was easy to walk her back in. The ducks seem to be out foraging more than the chickens. They do eat grass. They don't seem to suppress the fly population in the yard.
@poulet0heureux, regarding coop moisture: Much of this past winter, our coop was way too humid with ~10 chickens. Two 4" round holes, high and low, heavily screened, were totally inadequate. Eventually I opened one of the 2'x4' windows and conditions improved immensely. The bedding started keeping itself dry. No ill effects on the birds. I think that might be more ventilation than is really necessary and will investigate compromise configurations for this winter. I guess it was pretty ridiculous to insulate the walls.
