Because my family owned mutts until mom rescued an English Springer Spaniel from death row, I can't offer guidance in picking any particular dog from a breeder, rescue, or shelter. However, I will share our experience when DH & I searched for a dog to adopt.
We both wanted to adopt from a shelter or rescue. First we looked online at a few rescues in the area, but they weren't breed specific and didn't have a dog we both could agree on. We looked at the pound and found a few GSDs. 2 were a bit older, which we didn't mind, and one was a gorgeous black one taken in for cruelty/abuse. Well, the 2 older ones snubbed us and the cruelty case was so nervous that he couldn't focus for a second and just kept running circles around us. Granted we have no kids, yet, but there are lots of kids in our neighborhood. We decided to keep looking.
We went out of town and looked in a few other shelters, then we came across Peanut. She was listed as a shepherd mix (we assumed GSD mix) at a humane society. She was 3 months old, very quiet, laid back, and I loved her eyes, she was very attentive from the start. We adopted her, and it was the right decision, for us. She fit into our house immediately, loved playing with the cats, loved being with us, and in time just seemed to protect us as if she knew something wasn't right. She'd run to the door barking, or onto the patio when she noticed something didn't belong. Our vet listed her as a Rottweiler mix. Now, I didn't mind having a protective dog, but did NOT want an aggressive dog and we decided socialization was mandatory. Once she healed from her spay surgery, about 1 month, we started taking her to the dog park to socialize, then signed up for a basic obedience class.
As Peanut grew, she really did not resemble a GSD much, and I thought the rotti was coming out in her. Turns out that she's an Anatolian Shepherd/Rottweiler mix (or the designer dog name: Rottatolian ) . From what we've read, the Anatolians are livestock guardian dogs that will protect whatever they are raised with; ie: if they are to protect sheep, they must be raised -sleep, eat- with the sheep. They are independent thinkers, in that they come to know what is normal, what isn't, and are ready to protect their domain, without any formal protection training. As a matter of fact, the literature, and a few websites, state and reinforce, absolutely NO shutzhund training! Peanut is exactly this, she love us, her family, she's mellow, friendly toward people that DH & I have accepted (she sees us shake their hands, she's OK with them). At the same time, she sticks beside us, sits and observes and because she's a stocky 80lbs, people are cautious around her. Our friends know her, so they're OK. She loves to lay in front of the patio door and keep watch, if she hears or sees something unusual, she BOLTS onto the patio barking mad. That's enough of a deterent for anyone who knows better, if they want to try to enter my house, good luck! The only bad experience I had with her was during her "terrible two's" she decided to patrol the neighborhood, just like a livestock guardian dog will make it's rounds around the herd. Funny though, every evening when I got home from work, she'd be sitting at my door step waiting for me. I know she got out because I found the holes under the fence. Miss Smarty-Pants!
The GSD that a friend rescued (est. 6months old) & we adopted was a nightmare, at first. We believe that someone got him as a "guard dog" and left him outside to guard the property at all times. He had no manners, couldn't walk on a leash, didn't know what toys were, and kept wrestling Peanut - sometimes aggressively. I lost my voice trying to teach him his name. I fought Draico every night for about 6 months to get him into his crate. He had no idea how to chew anything, he was nervous inside our house - I think he was lost. But, I'm glad to say I didn't give up on him, put the time in to train him (clicker and fanny pack FULL of treats) because he's turned out to be a great dog! His neck wound healed, I house broke him, trained him to walk on a leash, taught him to play wiht toys, he LOVES to fetch and search for his tennis balls. Poor thing also had a couple of bad cases of Atopy last year, we worked through those issues,too. I don't think he was fed the best of foods, my guess is Ol' Roy. I got him on a better kibble (Royal Canin Maxi GSD, now on Canidae), and started feeding home cooked, along with fish 2-3 times a week, and yogurt and fish oil nightly; bathed him with a coal tar shampoo to help the itch. This dog would lick me to death every night!!
One evening, it was dusk, we were going into a PetsMart, there was a guy beside the door smoking a cigarette. Well, Draico decided he didn't like something about him and started barking like a mad dog! The man walked away. Draico kept barking until we entered the store and I got on my knee and called his name to get him to focus on me. Once he looked into my eyes, I praised him for looking at me, he was a happy-go-lucky pup; tail wagging and sniffing the store. Peanut barked loudly a couple of times, and stayed by my side. Her tail was wagging once we were in the store and I patted her head thanking her. A young man at the register asked me if I was OK? I was SO proud!
Both dogs are velcro dogs inside, but outside - love to run, play fetch, search, walk, swim. They come when called, know basic commands like sit, stay, heel, etc. We're continuing advanced obedience. And they understand 3 languages, I love talking to them as they sit there twisting their heads listening.
While neither dog is protection trained, they are first and foremost a deterent, and I'm happy with that. However, if they sense a threat, they will bark aggressively and stand their ground. Now if anyone tries to get past them at that point, I pity them! To end, you can get a nice dog at a shelter, but I'd get one no more than 12 months old so that YOU can train it the way you want. I believe the young ones are much more impressionable and moldable, but be prepared to put in the time to train. Now, I now people who rescue older dogs and they make good companions, however many cannot be around cats, or children, or other dogs because they spent their lives as the only pet. Having children, I'd adopt only young dogs from a shelter, even a rescue - JMO.