I know a lot of folks say guineas make good "watch dogs" but mine weren't reliable in that aspect. They made the most noise calling to each other, and to passing satellites orbiting the earth, and not when they saw a predator or visitor to the yard. They did have an angry alarm sound they'd make occasionally when they'd see a hawk, but so do the chickens.
I started with 5 guineas & now have only 1. She hasn't been saying much since her last flockmate died. When there were 2 or more they would always roam the yard together. This remaining guinea is now left wandering around by herself, looking like the last Dodo bird on the planet.
Maybe it would be different if a guinea was raised alone from the start with chickens, it would be oriented more towards those birds & not miss its own kind so much.
But I would recommend guineas only if you like guineas & can bear the noise they make. Try to find someone in your area who has adult guineas & listen to them for a while.