I wouldn't expect a staffy to do anythign as a guard dog, I've met a lot and all of them have just loved everyone.
"OH HI! COME GET IN MY CAR, LOOK AT ALL MY STUFF. YOU LIKE GAMES? ME TO, LETS HUG!"
Harry is a cattle dog mix (but BYB ACD temperment through and through) as a guard dog he's highly reliable, not a single person is getting by him. As a family dog, he gives the best hugs ever, one of his favourite tricks is to lie on your chest with your whole throat in his mouth (he likes to "hold" you)
with family he's bombproof, with everyone else he's over reactive and suspicious.
Sophie was probably our best example of this sort of dog, she loved people, loved kids, loved puppies or any other baby animals. You could take her anywhere. But when she was young, every evening at dusk she'd do a lap of the perimetre of the houseyard, tail up, nose down, checkign everything was in order and then retire to the house for the night. She knew when to put herself between us and something dangerous (she once leapt between my sister and a dog that was lunging at her, she didn't even fight, the dog backed down right away)
Sophie was a total mix, she was meant to be boxer/bullmastiff but she looked more boxer/dane etc. She was bred from pig dog parents but never had that attitude.