Sassy, you do know that French Guineas do not reproduce naturally right? They have to be artificially inseminated in order for the Hens to lay fertile eggs. So what you start with, and what's left over after the predators take their share is all you will have, unless you buy more. In my opinion, introducing new birds to an existing flock is even more of a hassle than starting an original one, just food for thought... you might want to start off with at least a half a dozen more than you think you eventually want free ranging your land.
Your brooding plan in the horse stall is a good one, altho if it were me, I'd keep them in until they are 12 weeks old tho (6-8 weeks is too young, they are still babies and should be eating grower feed at that point). When you do start letting them out to free range I'd start with just short periods in the evenings at first, being sure to consistently call them/herd them back in before dark (before they fly up into the trees) in hopes of establishing a nightly routine of going in at night. I've found with my own young flocks, that taking their food away around noon before letting them out later in the afternoons/early evenings helps bring them back in too, besides offering them treats.
Best of luck!