I have mudcovy and pekin drakes. This year it was the smallest--the pekin drake--that was bullying one of the two largest muscovy. As it was one bullying, I separated him at night into a dogcrate in the coop. During the day, the bullied drake could stay in the coop, or go out foraging with lots of hiding places, or his favorite was to go and sit in top of the compost pile occasionally standing on the pallet at the back of the pile to keep a look out for the other drakes. This went on for months. But it is all back to normal now with bullied drake in with the big muscovies and the bullying pekin hanging around but just outside side swiping pecking distance. He remains in his crate at night as he may not be bullying now but he is very annoying to the muscovy. I am afraid he could be injured if he annoys them at night.
As all your ducks are participating in the bullying, I suggest you partition off an area in your duck house or put a small dog crate in the duck house for the little one to sleep in. Give the little one food and water inside the night area to be sure it is getting enough to eat. By all means separate an area for it to have safety during the day if there are not places for it to hide. I also think getting a friend for the little duck would be a good idea