Do you have to train a dog to be a "watchdog" or is it breed instinct?

My family doesn't want a viscious dog but we thought it would be nice to have a dog who might be wary of strangers and bark at strangers. We read through 3 books from the library on dog breeds and adopted a rhodesian ridgeback/boxer mix pup from the shelter because the books said that RR were also used as guard dogs. We do take Toby (the pup) to parks and my dad takes him to our farm and other places where there are strangers, so he gets a lot of petting, which could be the problem. We also shower him with love so maybe that's why he is so sweet? I know how much you all hate stories but I must explain what happened yesterday. I live 1/4 mile from a road, fenced in gated yard, 3 other houses before mine, in a rural area, yet somehow we've had strange people show up. I was outside training my dogs and a guy with a mohawk, entire body in tattoos, a bloody lip, and a black eye showed up telling me his girlfriend tried to kill him and my puppies wagged their tails like crazy and the RR pup was licking his leg. I went into town for an appt. came back and was again playing with my dogs outside and he showed up again and asked me for a ride, at this point I called my grandfather who has many guns and came over. But once again, my puppies ran up to him wagging their tails!! It would have been nice if they would have growled at him or barked!!! My friend said you have to train a dog to do this?????? Please no snoody comments. Please answer as an adult, maturely. The RR/boxer pup is only 5 months old. The other pup is just too sweet as well, but we love her.