To start off, you have an adorable pup!
I am not aware of a previous thread from you, but it sounds like they are recommending no walks until your pup is four months due to vaccinations?
There are split opinions on exposing puppies to the outside world before they have all their vaccinations done. While it is a risk, I am of the mind that the risk of being infected by something is far lower than the risk you take not socializing your puppy to new people, environments, and stimulus at this time. Now, this socialization should be done in an intelligent manner. Of course, only socialize with familiar dogs that you know have vaccinations, only allow people to interact with your puppy in a respectful manner (no children manhandling your pup), and avoid high traffic areas such as dog parks that will have a lot of feces your pup could get into which would heighten the risk of infection and disease, especially parvo. Make sure interactions are positive and give your pup a break if it looks like they are getting overwhelmed. Vets often take the "better safe than sorry" approach when it comes to a pup's health, but socialization is such a big deal, especially with a GSD, that smart socialization that minimizes the risk of infectious disease should be carried out, IMHO.
As for Science Diet, the Hill's company funds almost all vet schools and veterinarians are given minimal knowledge in pet food nutrition, and what they do get is largely a salesman's plug to recommend Science Diet. The Science Diet Ideal Balance is alright, albeit overpriced. I would do some research as far as foods. The basics I tell people to look for are absolutely no by-products, corn, soy, or wheat. They are all just low-grade filler ingredients. I am partial to Fromms (especially for puppies), Merrick, Wellness, Orijen, and Castor & Pollux. My pup is currently on Orijen and is doing spectacularly, though what food they do best on depends largely on the individual dog.