I've a little 8 pounder who gained some weight / muscle after I got my GSD because the GSD gave the little one good daily workout plays. =)
#1 advice - be fair to both dogs. If GSD is irritating the small dog, address the GSD immediately and consistently and if a pattern is developed, watch for the trigger and address it before the GSD start the irritation. Same goes for the little one. It can't be one sided. I made the mistake of only addressing the GSD at first but once I addressed both dogs' behaviors I got peace. No one bullies the other. I set the tone, they stop whatever they are doing to each other when I say so. Both dogs must be patient and polite (learned that from Patricia McConnell on pack structures) towards each other and me. For example, if I'm petting one, the other cannot push in. The other must wait (how you do this depends on your dog and circumstance). One cannot steal toys or food from the other. With consistency and firmness on developing a fair environment I think it'll take care of a lot if not most problems of having multiple dogs. I have three dogs and enforcing a patient and polite environment keeps me sane and the dogs from bickering with each other.