Guineas are a ton stronger than chickens and much faster. Their genetics for survival are still those related to living in the jungle which can be vicious.
A larger flock of Guineas usually works better where chickens are concerned because they have high enough numbers to focus their aggression on each other. Luckily the hard hits a Guinea takes from another Guinea can be taken since they are built for it. Not true for a chicken.
The other issue where Guineas are concerned is that when they take a disliking to another creature they move in and attack as a flock. I've watched them run off fox, deer, cats, other large birds more than once. If the creature that they are advancing on doesn't get the hint then they will and do attack. They had one of my roosters cornered a few years ago. Yes, they. They decided he didn't belong and went after him.
So all this business of yes they can live with chickens might be true for a few but it is not the norm. Everyone should be prepared to have other options if they attempt to house Guineas with chickens because when it goes wrong its bad for the chickens.
Read through some of the posts here, people looking for help because their Guineas are attacking their chickens. You will find quite a few.