How old is your Pekin? Have you tried to correct the wry neck? Always when adding new ducklings to already established flock whether one or many ducklings should be kept on one side of fencing adults on the other so they can meet and greet. It can be a temp setup inside a run and coop or in the house but only with supervision should ducklings that are new be with older ducks like Kkrista said they can be too rough and injure.
I don't know why Helloworld would say Pekins are mean mostly unless they had a bad experience, because they aren't. They are very popular as back yard pets because of their sweet nature. Drakes of course can be hormonal which is another reason drakes shouldn't be around ducklings but for the most part Pekins are very sweet natured.
My chickens and ducks and geese are always together and never have they passed on any sickness from one to the other.