My first reaction is to suggest you use extreme caution. It would only take one chomp.
True guard dogs are highly trained, and their owner/handlers are highly trained as well. If that's not the case, I'd at least get a second opinion from someone who is more experienced. Have them test the temperament of the dog and maybe supervise the first interaction.
I have my little dogs around big dogs all the time, so I'm not paranoid by nature. I'd just want to know much more about the guard dog.
Here's the site I Googled. Not great, but some good information:
Guard Dog Training | Family Watch Dogs | Protection Dogs | Schutzhund Training
Here's one part that stood out to me:
The Dog Risks: Trained dogs are highly sensitive to strangers, sudden movements, high pitched squeals or screams, etc. ANYTHING that can be perceived as a threat may set a trained dog to attack - like children screaming, someone holding a raised object, loud yelling, someone moving too close to an owner, etc. It depends on the dog's temperament and nature, but not all trained protection dogs are rock solid on temperament and always make accurate decisions. This is why the vast majority of retired police dogs and military dogs are only ever placed with the former handler or former K9 police or military officers.