I also have Muscovies in with chickens, and they do fine. In fact, I have a little bantam Cornish hen in there who sits eggs along with the ducks. She gets right in the nest with them, and claims some of the eggs for her own, and when the ducklings hatch, she mothers them along with the Momma duck. If you're sure the Scovies did the killing, then something really strange must have happened to have upset them that much. So sorry for your loss!