Thanks for your question about guard animals.
I do not have experience specifically with donkeys
but I do have an opinion about optimal guard animals
for alpacas. The very best option in my
opinion for many reasons is a llama. Llamas
are no match for a mountain lion but neither is a
donkey. Llamas are very compatible with
alpacas; they eat the same thing, poop in the same
dung piles, and provide a wonderful feeling of
leadership and security for alpacas. Llamas
may speak a different dialect but they speak camelid,
and I think they get along better than guard animals
of another species. Donkeys, while lovely
animals, will be a challenge to barn management
(random pooping). They are noisy and could
hurt an alpaca with a well placed kick.
Keeping in mind compatibility, cost, and
effectiveness, I think a llama is the best option
for general security.