I've got 15 hens and 9 ducks living together (including drakes and cockerels) ... they get on pretty well actually!
Generally, the hens are the boss of everything, they eat first etc. The ducks don't seem to mind too much.
Although they share the same shed for sleeping in, it's divided in half, so the ducks have the bottom part, and the nest boxes and roosting bars are in the top half - keeps everything seperate.
The ducks do try and swim in every single container of water, regardless of size, so I have a 'proper' drinker for the hens, that the ducks don't use.
The run at the moment is a mud bath, however, in a couple of weeks, they're being moved to another part of our field. I find that as long as the hens have somewhere to go that is out of the mud - log pile, branches etc; they are fine and don't really mind.