Not too concerned with the chickens right now..blackhead would not present like this.
Feather eating is not all that unusual during moult but it could mean a deficiency of some sort. Definitely try the vitamins as a start. Also, if they'll eat black oil sunflower seeds...that may help. Make sure the turkey feed is not medicated (thiamin blockers, like amprolium, can produce neuorological symptoms if they are on it too long). I would check the bird over for injury as well. I'm leaning more towards injury, but nutritional issues can cause these types of symptoms.
For your mixed flock, I would recommend switching them all to a Flockraiser feed if possible. If you have large fowl chickens, get the pellet form, which is guaranteed not medicated. Our crumbles here are medicated, so you 'd have to ask if they are there as well. It provides better protein than maintenance feeds for turkeys and chickens.
Jody