Guineas and chickens both have feathers and lay eggs. In my limited experience that is where the similarities end. Guineas are crazy animals. The only “protection” they add to my flock is their warning yell. They will scream if another strange animal approaches, like a fox or hawk, but they are usually too busy looking for food to be the first to notice the danger. They will also scream if a dead leaf falls out of a tree and surprises them. I feel having a single guinea is a terrible idea.
As @R2elk has stated, they are a flock animal. What he doesn’t explain is they are a flock animal in need of their own kind. They do not want to flock with other species of birds. Mine coexist with my chickens but only grudgingly. While they really aren’t “terrorists” here, they are definitely at the top of the pecking order. They are VERY dominant over the other birds. Our alpha rooster is at least twice their size, but has learned not to try to boss them around.
Supposedly guineas will protect your property from snakes but you have to have several of then before they will get brave enough to attack one. Mine will show interest in snakes but have never tried to kill one. Mainly I realize a snake is in the yard because they are standing still staring at it.
One guinea hen is not going to protect your little chicks. Their eggs are delicious but they only lay a couple times a year, at least around here. Trying to raise guineas and chickens together is a challenge to say the least.