I run my 2 muscovies hens, 2 female geese, a pair of Guineas and my poulty of variying sizes together with very little problems.
The guineas and geese can be mean with the odd hen (pulling out feathers) but it only happens when the group gets fed and once spring/summer hits and everyone is outside, I've almost no spats between the birds. I have not (yet) had any casualties or blood drawn - some missing feathers from time to time, but nothing serious. The gunieas were raised with the chickens as day olds, the geese and ducks were not.
I did have another pair of guineas years ago that I bought when they were a few months old and had no issues with them either running with the chickens, even though they had never seen a chicken before I brought them home.

Muscovies don't need a lot of water, just a bucket/pail to dunk their heads in. They stay clean as long as their water is changed everyday and they can splash it around - my one 'scovie hen refuses to swim in the bath tub provided and only uses a small water pail for a 'sponge bath'.