I mix the pepper into the feed and ACV is apple cider vinegar which goes in their water. The theory from what I understand of it is making the gut of the chicken so miserable the worms cannot live there. I also live in a place where the poop freezes solid for 6 months of the year, which might help.
I have only had turkeys for a year, but I have had chickens the entire time I had turkeys. I knew going into it there was a chance of blackhead. I own the same farm I grew up on in the 50s, MY folks had turkeys and chickens together with no problem. There was an old veterinarian that owned a turkey farm about 2 miles from us and he had turkeys and chickens. He swore by giving the turkeys vodka. I would guess it was the same theory killing or driving the worms from the gut.