Well, guineas won't keep your chickens safe from hawks. They'll squawk at everything, sure, but guineas will be taken by hawks as well. They don't actually protect anything.
It only takes one Guinea to sound the alarm loud enough to alert your chickens there's a hawk... but it's up to your chickens if they want to run for cover or not, lol.
I have two turkeys and they are kinda dumb...but umm I was told the guineas would help and goats would not help? Also that big roosters would help? Hmmmmmmm
Whittni, I have never had a predator loss, but I do credit my awesome roosters and my tree cover (I live on a small mountain in the woods of N. Ga) to that. Roosters will do just as good a job at watching the skies as a guinea will, IMO. I loved having guineas, but they do screech at everything and nothing. As far actually protecting, a rooster will try to protect, in that he will charge and take on a predator (though will most likely lose his life while the girls get to safety), but a guinea will not "protect" the chickens as a rooster will attempt to do. They are pretty much just predator alarms times ten. Guineas are very entertaining, to be sure. They often will not get along with chickens, and I have the first hand experience to prove that. robin416 can elaborate on that for you as well.
I think maybe you would have to try your hand at guineas to know. A larger group that lives separate from the chickens is really the best way to go. They'd free range the same area and their alarm calls would also alert the chickens to danger, but it just depends. It's hard to say how guineas would work out for you until you have them. They are nothing like chickens, trust me. They wander farther and in a more methodical manner, like an army doing maneuvers. They attack/surround something as a group, if they aren't running away at 90 miles an hour, but it may be an empty soda can or a frog or just anything, really.
guineas need lots of room during the day. My coop is 18x10 and I have 14 guineas and 2 silkies. They free range during the day in my run. The run is 50x30 and they have it clean of all bugs and most grasses.