Breed isn't all that important. It's more about the time you put into training and the individual dog's personality. Protection duty is one of the hardest things to ask of a dog. First you teach basic obedience while working on socialization, then you work on agility training, and after that's done you can start teaching it bite work and counter attacks, -- that's if you really want a protection dog.
However this requires a smart obedient dog with confidence and a high prey drive. You can also expect it to take at least two years to train your dog right and it's better to get them when they are young. Otherwise you'll need to work undo all the mistakes from their last owners.
I think it's great your looking at rescues though. I got my pup from a rescue when she was only eight weeks. I put her in puppy classes right away and made her an inside dog. I see lots of people bringing in Pits and German Shepherds because they wanted a family pet / guard dog. Most of the time these dogs have a mean bark but they are really just big babies and are only barking out of fear. All it takes is a few barks from a really confident Chihuahua and these dogs run behind their owners with tails tucked behind their legs.
Ideally, you should visit the dogs in person and play with them. See if they like to play fetch, tug, and will come when called. Get the dog that you naturally bond and are willing to keep no matter what. If you can, make them an inside dog and hand feed them for the first couple of months. You can always teach them to go outside, do a quick patrol, and bark at intruders.
