I don't have ducks,,,,,,, but let me still chime in with ideas. Are the pulled feathers from the drake trying to mount the Ducks??? If so,,,, then maybe consider something like chicken aprons,, that protect hens from over-aggressive roosters.
If it is other reason for the feather plucking,,,,,, maybe the mallard is not compatible with your other breeds..

I do have wild mallards that visit my yard during spring, and summer daily. They eat the seeds dropped from bird feeder, and drink the water I place in containers on ground. They are always paired up with a hen,, and I never have seen any rude behavior.
You may have to rehome drake, or figure a different alternative to him not being with your flock any longer.
WISHING YOU BEST,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,