Long winded post here...
You will get different opinions on the snake killing, typically it depends on the Guineas, some flocks are way more passive and lazy than others, some are super aggressive and watchful. I personally have witnessed my own Guineas killing small snakes (playing keep-away and tug-o'-war with them 'til there wasn't much left of them but something that resembled a bloody string - eww!) and I have also witnessed my Guineas circling around a large rattle snake, raising a complete ruckus (so much of one that my dogs started howling and my Donkey started braying) and then they were pecking and clawing the snake from every direction until they had just about turned it into snake sushi and ran it off my property thru the fence... (then they continued to yell at it from our side of the fence for another 10 mins to make sure it didn't come back, lol). I have not
yet witnessed them actually
killing a large snake, but I am sure that one went off and died, lol.
I start letting my young birds out at around 12 weeks old, when they are pretty good sized and have a little grey matter upstairs and are more alert about their surroundings and predators, but like livenwithpeeps I have existing adult flocks that they get integrated into and they learn the ropes from them. From the start, when they are keets I always use the same call at feeding and treat time, so I can call my birds from anywhere on my property and they will come running/flying to me knowing I have food for them (it's kind of fun to have friends or keet/Guinea customers at the house and then call the birds to me... people are always impressed and say things like "my dogs don't even come like that when called!", lol). I also use that same call to get them in at night (and at approximately the same time), and they always get food and treats when they come in. I begin their free ranging routine by letting them out hungry at the end of the day for just a short period at first (I usually take their feed away around noon), then closing their pen while they are out, follow them around while they are out, not letting them go too far and then calling them back (with the call they are used to) at feeding time, usually right as the sun starts setting. I gradually increase their free range time and distance each time until they are reliably coming back in when called, then after that they get let out in the mornings and can stay out all day. It helps to have someone helping you herd them in each night until they get the routine down and start coming in when called. Guineas like routine, once you get one established, (but you have to be consistent and establish one for them, or they will establish their own, like roosting in the trees or on the rooftops instead). I know this is a lot of work, and not everyone wants to or has the time to use this method, but it does work and has worked for me for years.
With the exception of hawks and owls it does not sound like you will have much of a predator problem with your Dogs and Donkeys being on duty, IMO your main issue will be getting them to come in at night to roost in the safety of their coop/run so the owls do not pick them off one by one if they choose to roost in the trees or on rooftops. (And hopefully you do have a roosting place for them at night, because typically a flock of 12 will quickly become a single digit number, since predators tend to return for a free meal night after night as long as they can). I have sold lots of keets and Guineas to people that just turn them loose without working with them and training them to come in at night, and these people usually come back once or twice a year to replenish their flocks, regardless that Ive already warned them they will lose them one by one to predators. So I highly recommend that you provide a safe place for them to roost at night, establish the nightly routine of coming in and also training them to come to you when called, whenever they are called.
Guineas are a lot of work at first (especially to maintain a flock without losing any), but as they get a routine established and they learn the sights and sounds of your land they get a little more easier and easier to deal with as time goes on. Hope this helps, and good luck with your flock