They will go through cattle panels easily, but they aren't going to roam very far, you don't have to worry they would run off into the distance. You do have to worry about predators, especially if they wander out of the donkey pasture. Whether the donkey will protect them either purposely or inadvertently, I don't know, but if so, it is probably limited to ground predators, maybe just to dogs and coyotes, but not hawks or owls.
I had a donkey years ago that bonded with my guineas, so there is hope that there would be a relationship that could benefit the chickens. If they eat bugs off the donkey, then the donkey will enjoy their company.
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I think the color pattern of the speckled Sussex or any chicken is of less importance than its temperament in regard to vulnerability to predators. Some of the more naturally flighty breeds are more savvy about evading capture than heavier friendlier breeds like the Sussex. That said, my Speckled Sussex going on 5 mostly free range years, is the only one of my older hens left. Was it luck, camouflage, savvy, or a combination that protected her?
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I'd say go ahead and try out your plan with the fencing as is, and see what happens. Plan on raising replacements each year for any lost to predators. Have a secure run ready if the losses are too high in your area.
I will be interested in hearing how it works out---good luck!