Wow, I have the exact same problem as the OP. Two guineas, not quite a year old yet (both male I believe), that have free ranged with the chickens since last summer. Suddenly they have turned on one of the roosters, and basically defend his hens from him as if taking over. I even saw one try to mate with one of the hens! So it seems apparent that they need a flock of their own. I don't want a huge flock, so bought three babies (now 5 wks old) hoping for at least one female they could focus their attention on. Right now I'm trying to determine if it's only one of the two guineas that is the problem - once the chase is on, it's always just one guinea after the rooster. Unfortunately the guinea is faster. Either way, I plan to wait it out until the three new guineas join to see if that helps.