Q: How do working line GSD tend to do when they interact with other dogs? A dog park for example. given other dogs are neutral (no behavior issue, etc). Do they tend to be more assertive toward other dogs? dominant? reserved? care little for other dogs outside it's pack? playful?
I think it really depends on the dog. Halo is our first working line shepherd, and she was in daycare twice a week for a few months when she was younger, at a boarding facility that we use when we're out of town. My reason for that was partly to get her used to being there as a fun place, so when we did have to leave her for boarding it wouldn't be brand new. This particular place has private rooms for the dogs, and puts them into play groups based on age, size, and activity level. The dogs have several play/potty breaks outside per day, and then go back to their rooms, it's not an open free for all kind of place. The staff loved her, and reported that she had several "friends" that she enjoyed seeing each week, and very good play skills.
Another reason was to separate her from our other dog, Keefer. We wanted them to not be too bonded to each other from her early weeks and months, and also to learn good social skills without him around to depend on. When she was 15 weeks old she started going with us to off leash parks. These are large multi-acre places where people walk around with their dogs and play ball, not little fenced parks where everyone stands around and watches their dog play with other dogs. We have a tiny yard, so in order to play and get exercise we take our dogs to places where we can chuck a ball for them to chase, and to go swimming in the SF Bay and the ocean. We don't go there for our dogs to socialize and play with other dogs, they play with us and each other.
Far from ending up being too into other dogs, Halo is actually the most neutral GSD we've ever had. Being off leash around lots of other dogs was never a novelty for her because she was used to that situation from a young age. At the park, other dogs are merely an obstacle to run around to get at her ball, like a bush or tree. She does greet her flyball teammates with a sniff and a tail wag, but other dogs she's just not that interested in. We sometimes get together with other GSD people in the area and go on hikes or to the beach, so pack walks aren't a big deal either.
Although Halo is my sport dog, our dogs have always been first and foremost our companions. She's wild and crazy and loud racing in flyball, but has a terrific off switch so she's really easy to live with at home. I don't think what you're looking for it out of the question, by any means. A dog that's not that interested in other dogs in favor of being more focused on you can still be perfectly comfortable walking in a pack with other dogs, and even become friendly with some of them. Socialization is about exposure, your dog doesn't have to actually play with the other dogs in order to benefit from the experience. I understand it's not what everyone wants, but I personally enjoy having dogs that are friendly and social with other people and dogs, so when my friend comes into town with her dogs we can meet at the beach and walk and play with our dogs.
This is from a large meet up at the beach in SF from a few years ago, I think we had a total of 16 dogs, mostly GSDs:
And another more recent on on the same beach (one of these is not like the others!