Nope, Guineas aren't all dark meat... the breasts and wings while a little darker than a chicken or Turkey are definitely white meat, and noticeably lighter than the thighs and legs. Guinea meat is super lean too, so it can be overly dry depending on how it's cooked. IMO it's more like Turkey meat, in both flavor and texture. Guinea is actually substituted for Pheasant in a lot of restaurants (sometimes listed as "African Pheasant" on the menus), but I've never tasted Pheasant to be able to compare the 2. I have eaten Guinea quite often tho (and there's about a dozen in my freezer from this season so far), and they indeed are tasty. Just not much meat on them (they usually dress out at a little under 3 lbs). You may want to just process a couple to see if you like the flavor and if it is worth all the work of processing them to you or not, then if you decide it's not sell the rest of them (alive) on craigslist and buy feed for your chickens for the winter, lol.
How big is your coop and pen? Do the Guineas get to free range? The more space and the more free range time the better whenever Guineas are concerned. They can become very territorial and aggressive if they can't get out and run off their extra energy and hormones each day. Maybe booting the Guineas out of the coop each AM and letting the poor Hens have some peace and quiet to lay each day will help?