I keep my domestic guinea fowl with the chickens, they can and will pick on the chickens from time to time but they are all able to free range during the day and I have never had them actually hurt a chicken. I have always introduced the guineas to the chicken flock as soon as they are out of the brooders so they are not only used to the chickens but they are submissive until they mature, by then they have sort of a pecking order and the chickens tend to get out of their way so it isn't a problem.
I have a 8X8 ft pen at one end but still inside the chicken run (which is 40X10 ft) that I put any bird I am raising in so the flock gets used to seeing them daily, as they get older and are about 2/3rds the size of the adult chickens I slowly introduce them to the main pen, first for an hour then longer before I let the adults out to free range. Finally I let them out into the pen for the main part of the day while the chickens and guineas free range. After a couple of weeks of this I let them out for the day with the other birds kept in the pen for another 2 or 3 weeks before I start letting them all free range. This allows the younger birds to learn the barn, pen and feeding/watering stations without being picked on or driven away until they are finally integrated into the flock.