I just had half of my chicken flock taken out by a fox. I've read that guinea hens can help protect your flock. The problem I'm running into is sellers want huge orders. I have 12 chickens left, I don't need 30 guineas! Also, another obstacle, we have Marek's. My last batch of chicks was vaccinated and they're doing well. But do any hatcheries vaccinate guineas? Thanks!
Guinea are more "alarm" birds than "guard" birds: they're much more alert and paranoid than chickens. Depending on a variety of factors they may or may not get along with your chickens. Mine do get along about as well as poultry does, but they free-range most of the time and have separate sleeping areas.
How well their "alarm" calls alert your chickens depends on your chickens. Most of mine are pretty good at paying attention to their guinea "friends" when they sound the alarm.
The problem is that the guineas always travel in pairs or groups, and one of them usually acts as "watchdog" for the others. The chickens don't do that. Often one will wander off on its own, and those are the ones that usually get grabbed in daytime.
To date the guineas are extremely effective at protecting the flock from airborne predators, but have mixed results with ground predators. Against raccoons or opossums they spot them quickly enough to ether get away or give me enough time to deal with it before it gets anyone. The noise they make keeps coyotes away IF the fence is intact.
They aren't very good against ambush predators, as they tend to be unable to spot the threat until AFTER it strikes. I doubt they'll help with your fox problem.
Also, they see even worse in the darkness and dim light than chickens do (I have a couple that are pretty much blind in dim light). So unless you have lights in your coop / run they'll be even more helpless than your chickens if something creeps in at night.
As far as mareks goes, to my knowledge they're much more disease-resistant than chickens which is probably why the hatcheries don't bother vaccinating them.
Downisdes: Guineas are flighty, pushy, LOUD birds. Even if you imprint on them they'll usually try to avoid you & hate getting picked up, they tend to gang up when they have fights (which is going to be lethal to a chicken or rooster that won't back down), and they are LOUD. If your neighbors complain about roosters, their heads will explode when your adolescent guineas scream their heads off at anything that upsets them (which is practically everything).
They get more discriminating as they figure out what's an actual threat and what isn't, but when they go on full "red alert" the noise level at "ground zero" is incredible. I've worked in high-noise environments for 28 years and I find it mildly disorienting. Even as adults there's times they just like to make noise: my flock "sounds off" for about an hour after sunset, and they also like to chatter a lot when it's a clear night with a full moon.
You will need more than a couple of guineas to have proper social dynamics, but that being said there are hatcheries that will sell you far less than 30 at a time.