They may get along until they approach mating age, then they may turn on the chickens. That's why I was forced to rehome four very tame guineas who were pretty much raised in the coop with my hens. Alpha male began attacking one hen, then it progressed to all four guineas attacking the other three that looked just like her (RIRs). Later, they turned on my BR rooster and I couldnt take it anymore. They act nothing like chickens, they attack like wild gang members, etc. They just are not chickens.
People may think they're like chickens, but guineas know who they are and act accordingly.