This is a 10 month old pup. There is still time for the pup to display his/her guarding instincts. As stated dogs develop or mature differently. While it may be typical of certain breeds to begin displaying their guarding instincts by a certain age this is not going to be true for every dog. Some dogs mature more slowly. Some begain to display it early.
Just because a couple of dogs on the other side were great guardian does not automatically mean that all of its offspring will be great guardians. If that were true every dog from a "working" line would all be great schutzhund dogs. Every dog from a so-called "show" line will all be champion of record. Thesed dogs are often bred having working or show champions on both the Sire and Dam in the pedigree. But they both will produce more dogs that are not going to be great schitzhung (IPO) or Show champions. In fact many are going to produce some pet only quality pups.. The dog may still be a great dog. But it might not meet the criteria to be a champion of record or a good working dog.
I don't recall if I asked this previously. I kind of think that I have. Have you had the dog evaluated by a professional? This may give you a better ideal on whter or not this dog will meet your need for a guard dog.