Welcome to BYC. Glad you decided to join our flock. I'm a chicken person and don't know much about Guineas (never had any), but definitely post on our Guinea section in Other Backyard Poultry under the Forum at the top of the page and take advantage of our Guinea experts there. Please feel free to ask any questions you may have. We are here to help in any way we can. Cheers.
Guineas can and do free range and can do so well at it that they need very little feed in the summer. You will have to supplement them in the winter. However, they don't have ANY defense other than escape from both daytime and nighttime predators if you let them roost outside of a predator proof coop.
Edit to add: unless you have a LARGE acreage, they will be wandering on your neighbor's property. Not all neighbors appreciate that.