Unless said Donkey has been raised with the small animals, like the ones you want to keep.
I won't recommend it.
Donkey's have a habit of stomping animals smaller than themselves. Mine loved to chase the chickens.... or anything smaller than her, and most do.
Also won't recommend a mini Donkey for guard duty.... their bray isn't going to chase predators away, they will just get attacked and or eaten.
Mine was attacked and I din't even have her for a guard animal. Cost me a big Vet bill. And the reason we built the night pasture.
I have known Standard and Mammoth donkeys that were killed by cougars and bears, while trying to protect their flocks.
Llama's and Alpaca's, depends on the animal... some will guard well.. others like to stomp or kill your smaller stock.. or not guard at all. It is very hit and miss.
With the Alpaca's on the smaller side, like mini Donkeys, is more like they will just get eaten.
I would highly, highly recommend a good fence as your first defense. Most folks here have seen photos of what I call the Night pasture. And it has kept large Black Bears and cougars out.