Personally, I think it is a not so great idea. Your 5 month old puppy needs and deserves to have all your attention. Giving the 5 month old that attention will be very, very difficult with another puppy in the home (who also deserves to have an owner that can devote the time and attention a puppy requires).
In order for it to have any chance of working, you would need to keep them separated from each other, except for brief supervised play periods. Training, socializing, feeding, exercising? You need to do all that separately. That is a huge commitment, which is why so many people have a hard time following through.
I offer this advice with all knowledge of how difficult it is to ignore the desire to bring another puppy into the home. But truly the best thing for you, your current puppy in your home and the puppy in the rescue is to pass for now and to spend the next couple of years making your GSD the best dog possible, with impeccable manners and solid obedience training and then bringing in another puppy/dog. You won't regret waiting. But you could very, very easily regret not waiting.