In my experience, guinea cocks are fine with chickens if raised with/near them.
Our first guineas we got as day old keats, raised them in the house for a few weeks, then stuck them in a cage inside the coop for a few more weeks. We let them out with the chickens and a rooster actually adopted them. They followed him around. To this day they do not chase him at all.
Our second batch of guineas we got when they were 2-6 weeks old. They had been raised till that age with a couple chicks in the brooder but no interaction with adult chickens. We kept them in the house for a couple days and then put them in the "nursery" part of the run for a couple weeks. We let them out with the chickens and they were fine.
Yes, our guinea cocks do chase the roosters a bit and run off the hens from the food every now and then, but overall they do little to no damage to our chicken flock. Whenever I see a guinea cock after a rooster it is because the rooster first started fighting with the cock. The cock chases the rooster, then leaves him alone. And do you know what? The rooster comes back for more. (Silly chickens, always wanting to get the last peck in)
BTW, we have 2 guinea cocks and 7 adult roosters. During early guinea laying season we had 3 cocks, but still we had no problems.
That's my 2-cents about keeping guineas and chickens together. However your problem sounds different. I've never raised turkeys so I can't really say much on that point, but if turkeys really are mean (Like I here they are) then it might be possible that some of that meanness rubbed off on your guinea cock. Or it may be that he's feeling misplaced and wants some hens of his own? I don't know, lol.
Do your roosters chase each other a lot? I know all roosters chase each other a little bit every now and then, but maybe yours chase each other a lot and he's just copying them?
I know these ideas sound stupid, but you never know with chickens, guineas, and turkeys.
I hope you get this worked out, good luck
My turkey hatched out a single guinea last year after some dog problems where her own eggs were destroyed and the rest of my guineas killed. She raised the male guinea. Once a week or so his "Mother" chases him around like she is going to kill him now that he is grown. We have a big big pen. The Guinea will target a chicken he doesn't like and torment it. Tonight he chased the rooster in my avatar all over the pen, not letting up and then attacking him when cornered. I rescued poor Rocky three times today. I am so fed up with this guinea, but I am wondering if it was from his upbringing, being raised by a turkey?? He was raised by the turkey mom and among chickens, I don't get it.
