Whether a Working Line GSD can be trusted around smaller dogs depends on the individual dog more than the lines of the dog. I have a WL GSD who just adores my cats (and vice-versa), and tends to them when they are sick or injured, while some other WL can never be around cats because they see them as prey to chase and catch.
I would try to find a breeder whose dogs are known to be social, and get along well with small animals, small dogs, cats, etc, but that is still no guarantee that the puppy you get will be okay with your chihuahua, and you may need to be ready to manage your household in a way that will ensure the safety of your little dog.
BH, OB1, TR1, AD
Rottweiler/Hairy Dog mix?? Shelter rescue
Gryffon Vom Wildhaus