Sound like fun, what part of the world are you? Do you know the predators in your area? How many birds are you going to try? What's your free range area like, fenced, open, timber, etc? Are you doing electric fence for day and night security?
Seems designing and building a secure coop should be the top priority with lots of ventilation and possibly 2 sets of laying boxes. Also you may need to divide the coop and feed AM and PM as I don't know if guineas and chickens can share feed or if they will share roosts but both will need all day access to the coop to lay. The chickens are going to need a good layer mix, other wise your going to possibly have weak / thin shells.
As to running them together you should be just fine, most people do. Guineas are very alert watch animals and love to hunt snakes and bugs but you'll probably want a big breed rooster to do the fighting. Probably be smart to start a thread in the guinea section of the site for info and experience from their view point.