Ducks are a little more vocal than chickens and quite a bit messier. That may be why you can only have one duck. And as for keeping ducks with chickens, if you introduce them properly they should be just fine. I know many people who keep chickens and ducks together. The only thing is you need to...
I have a pheasant who's top and most of her bottom beak got pecked off by another pheasant when she was young and she has done great! We feed her crumble feed since she cannot break down pellets. Hope she is ok!