I had heard you shouldn’t try to keep guinea fowl with chickens, so I didn’t. But our guinea flock was destroyed one bird at a time until we just had one. He was so lonely ... he went to the guinea shelter two nights in a row after his last buddy was killed, but then the third night, he went to his shelter, stood there a couple minutes, then made the big trek back across the field to our chicken coop (which was still open at dusk). He strolled into the chicken coop, flew up to a perch, and lived as a chicken for the rest of his life. We didn’t have any issues with fighting - he didn’t bother our hens or bantam roosters. A friend of mine keeps chickens, guineas, and ducks all together. So ... I am not sure why it’s recommended not to, but my experience is that it can work.