The biggest problem with crested ducks is the same gene that gives them the crest can cause a hole in the skull that kills chicks while they're still developing in the shell. As far as I know, once they hatch out, they're as healthy as any other ducks. They're fortunate that you were there to rescue them. <3
I don't have a lot of space, so a few call ducks will probably be all the more I do in the area of waterfowl. First thing I do when I take over the property from my dad is put up a fence to keep the birds (mostly) in the yard e_e Then I can think about digging a pond and getting ducks.