We have an alpaca Sol, who does a great job at protecting our flock and the lambs. He has bonded very strongly with the sheep, and is very vigilant whenever the ewes are lambing.
In our experience, the best kind of alpaca herd guardian is a wether who is 18 months or older, at which age they have developed the guarding instinct. Intact males are not suitable, as they are often inappropriate with their approaches to other species, (sadly often resulting in the loss of nuts). Two Alpacas per mob seems to work well as any more can form their own group. They do a good job in reducing lamb losses, but you need to be realistic about the size of the paddock and the visibility over the terrain. They can't patrol what they can't see. We have also found that our alpaca is used to our dogs but doesn't like them in his paddock. I think any alpaca that is too laid back about dogs might be suspect as a guardian generally. The husbandry rerequirements are similar to sheep and goats, but don't forget about them a good guardian is worth extra effort.