Are all the same age and come from the same place? I see a drake how is he with the Pekin?
It's great when they have some scuffles and then all is worked out but sometimes it just doesn't work and the poor duck that is being picked on is so stressed it can lead to sickness. Especially if she is bullied away from food and water. I have 11 yr old Muscovy females who have to go through time out because they can pick on one of their own. At time for bed I won't tolerate one picking on another when there is no place for the one being picked on to get away. I keep a large dog crate in my largest coop and that is time out overnight. Try and watch to see what is triggering this behavior. Possibly even the color of her feathers? Rouens being dark colored and the Pekin being white may have something to do with it. Separating the Pekin with the lesser bully might work to start to solve this let them be together for a week or less depending on how it goes, then introduce the other to them and see if that helps with the behavior. How old are they because hormones is what usually causes the bullying here