I love, love, love muscovies. They can be very sociable and friendly, and are very self-sufficient. They are great for meat production (except since you only plan on hens ...).
I've never had hens only, but then you won't have to worry about over-excitable drakes taking it out on chicken hens, possibly, if you know what I mean.
The biggest problems you might have are if the scovies tussle with the hens a bit over food. The scovies may throw their weight around, but the chickens should be faster. Most likely I don't think you'll have a problem with dynamics.
Mine laid 2-3x a year, clutches of about 12-16, then hatched them. I don't know what will happen if you take the eggs. They are pretty seasonal though so you might not get constant laying.
They will probably appreciate bathing water, that they don't get as excited to have it as other ducks. Mine were more interested in food than anything, and followed me around begging like little puppies. I love the little raspy sound they make, not obnoxious like some ducks.
I doubt they will actually protect from predators, but they can be very vigilant. If your girls learn to watch them for cues, they might be helpful. I'm not sure the chickens will do that though ... they seem much more attuned to a roo or dominant hen. I'm not sure I've ever seen chickens take cues from ducks in all my flocks, but then I always had both sexes of both.
Good luck if you go ahead with them.
