Just a quick note on "dominance" and "alpha dogs" which are terms used very loosely by a lot of people, especially to excuse rude canine behavior.
Dominant dogs don't pick fights, particularly with puppies. Dominant dogs are respected because of their innate carriage and dignity -- other well socialized dogs recognize true dominance and yield to it (which doesn't mean they won't play with them, just that they will defer to the dog in question). The concept is true in people to a certain extent -- as a child, there were adults that you had no respect for, and those you did. There were teachers you thought were a big joke, and teachers you respected.
Dogs that create trouble are not alpha dogs. If you're being warned that some dogs in this group are too rough with puppies, I'd be concerned that she's mistaking poor socialization and aggression with dominance, and I'd be worried about those dogs in terms of your puppy's safety.
There are owners who take a misguided pleasure in the "dominance" of their dogs, when it's actually really bad socialization and/or genetic defect. "Oh, he's just very dominant!" Bull. Dogs who go after 3 month old puppies have something wrong with them.
Personally, I'd investigate doggie daycare or a setting in which you can trust the socialization of the dogs your pup will be playing with in its formative weeks, preferably with an experienced eye looking on and interpreting the interactions. I agree with Tulsa Dan that your dog will form associations as an individual based on his own experience, but a lot depends on his own essential makeup. Part of being individual is that some will be far more sensitive than others, and a traumatic experience CAN be scarring at the wrong time for certain kinds of puppies. Whether that's limited to, say, never liking German Shepherds again but otherwise enjoying other dogs or forgetting the whole thing depends on how many positive experiences the pup has had and its overall emotional makeup.
That said, there's a period during adolescence when even the most easy going adult has had enough of the juvenile pup and gives him a good talking to. Well socialized adults will tolerate a lot from young pups, but the day comes when they figure they're old enough to know better and put them in their place. In that period, especially, I'd want my pup to keep the company of well socialized adult dogs who will send the message loud and clear without getting carried away.
Kathryn Saxon
Daisax Boxers