Sounds like you are doing very well with her and it seems like she is a good, typical GSD pup. Keep up the training and play sessions. Peronally I would not another dog until you have raised her for at least 1.5 years and only then add a male dog to avoid the two-female-problem.
Is the two female problem a very common occurrence or just increases the likelihood of in-fighting? I just don't want to deal with a male + female dog in heat. I don't mind neutering a male, but I have heard spaying a female *could* cause behavioral problems and urinary problems later in life, so am on the fence on this right now.