It's always best to keep in mind that we are the ones that have taken these animals and put them into an unnatural living situation and we have an obligation to manage them in such a way as to make life acceptable for them and for us.
Ducks and chickens are not natural flock mates, if you confine them together in the same small living area it would not be unusual to see some undesirable behaviors.
I say it's not his fault because he has a innate drive to reproduce. If he doesn't have much choice other than ducks, then he's probably going to mount the ducks. Even if you keep pushing him off with your foot, he's still going to keep trying, because that's his main function in life. You're not going to be able to override his basic instincts with reprimands.
I would not put in him with the rooster, hen and chicks, that's a sure recipe for disaster!
We all here go through this at one point or another where we have to decide how many birds we can keep and which ones, taking their comfort and our ability to handle them into consideration. Chicken math got the best of you and now you have 7 or 8 males to deal with. It stinks, but that's the way it goes sometimes. Your best bet is to rehome or cull down to one rooster unless or until you can get another coop set up. Adding one more hen is not going to solve this problem.