Guinea fowl are a nice complement to having a free-range poultry flock. for me, they act as an early warning system for airborne threats to the flock. Hawks, eagles and airplanes all get a warning screech from these air wardens of the barnyard, and the other poultry take cover when they call the warning. They aren't much into eating plants -- which means that that actually do better than chickens in the garden (not to say they're above lifting a tasty lettuce leaf though...) and they LOVE bugs and foraging. They have a range on my farm of around 3 acres; mostly they're hanging around the buildings and corral, but now and then I'll see them a couple of hundred feet out, foraging in the pasture.
When you're mixing different breeds of fowl, you have to watch the guineas and make sure that the other fowl have a way to get away. Once they've decided that a particular bird has annoyed them, the flock of guineas can be a relentless foe, and they can and will kill a larger bird if it cannot retreat. They are not particularly aggressive; on my farm it's usually the young roosters who size up a guinea fowl in their little rooster brain and figure that they are bigger and can beat it easily. They rooster doesn't really understand the concept of teamwork, but the guinea fowl do.
You can buy guinea fowl keets at several national hatcheries, or sometimes at feed stores in the spring. they tend to appear later than chickens, so look for them in May or June. They are very independent, and mine make a very good living eating the food spilled from the hog feeder and foraging of the vegetables that I feed the hogs. I do provide a secure roost for them and I keep an eye on them. I lost a guinea fowl earlier this year, in July, to an unknown cause; it turned up dead one day, but the other 7 have been doing well.