I would wait till she's older and well trained. It wouldn't take much for the gsd to hurt the chihuahua.
It wouldn't take much for ANYTHING to hurt a chihuahua they look like they could be injured by a breeze
But if you're going to get one, do make sure your GSD is 100% trained and totally zen. I never even let my pup near a chihuahua or small toy breed when they go to the dog park. They could really break a bone from a little playfighting. Can't imagine what a grown GSD do.