I've never kept ducks and chickens together in anything other than free range. However, I see no reason why you can't do it if the space is big enough. You might be better off just getting a garden shed, and then you can add perchs and something for a nesting box for the hens. An old wooden drawer or wooden box on the floor would do as a nesting box. And the ducks can live on the bottom quite happily.
I have kept ducks in one side of a house, and hens in the other. But the partition didn't go all the way up to the roof, so the hens would perch on top of that above the ducks. Never any problems.
I can definitely understand you falling for black Indian runners. I have a pair of white ones just now, but I've love some coloured ones as well. I think we might look for some mixed coloured eggs to hatch this year.
Beth