I've had donkeys for decades. They have done a decent job keeping most predators at bay here for the past 20 years. We recently put down the old mom donkey due to age related problems. Her 2 boys haven't been as good at going after predators. Maybe they are still grieving their mom.
I would recommend a female for guarding. The males can sometimes go after critters. Mine have gone after the goats, and muscovy ducks in their younger days. I haven't had problems with them and the chickens. I sometimes see a chicken riding a donkey. One chicken would routinely roost on one during winter one year.
Donkeys will stomp any canines so if you have dogs you need to be careful.
Donkeys are a long term commitment, but are fairly easy to keep. They don't eat much, and don't require fancy stuff like grain or alfalfa. Good pasture and grass hay will keep them fat, maybe too fat. They do need regular hoof care. Otherwise they are pretty simple.
They aren't like horses. So working with them is different. They are intelligent critters, who need gentle handling. They will never forget cruel handling.
If your set up is right they could help with predators. Not every donkey does the job well or consistently, so be sure you actually want the donkeys, yes get 2 or they will horribly lonely.