Yes, guineas are great at sounding alarms. I personally have never seen my guineas attack a predator such as a cat or a snake, but I have heard stories of guineas doing that. Mine will make a half-circle around the cat or dog and squawk like crazy though. As for guinea cocks attacking roosters..Roosters fight with each other, and it's only natural that guinea cocks would fight with roosters sometimes. However, if a young rooster picks a fight with a guinea then he's not too smart..the guinea is going to win. In my experience the guinea cocks simply like to let the roosters know that they are above them in the pecking order. ALTHOUGH my main alpha rooster was above the guineas. And they knew it. The guinea cocks might chase the roosters by their tail a little, but it's never been anything serious in my experience. And the only time they would normally do this is in the spring & summer. A more serious problem might be two guinea cocks fighting/chasing each other, in the summer heat. But for the most part they all get along nicely. You can keep hens without cocks, but for the most part although hens "buckwheat" almost constantly, the cocks are better at sounding the "alarm" I think.
I keep my guineas in with my chickens year round, sometimes it's a 1:1 ratio and I rarely have any problems. Worse case, the mean guinea cock becomes supper or finds a new home. Guineas have really added a LOT of personality to our flock. They each have their little quirks, and it's lovely to see the cocks being so attentive to the hens when they pair off in the spring.
