I can obviously only speak for my own dog, but he lives with an 8 pound dog and he does great. When he stays with his trainer he is with other large dogs and is very rough. When he gets back home he is able to switch right back to being gentle. My GSD is close to 2. Both my dogs are male - my GSD was unneutered when he first started living with the small dog.
It took quite awhile for me to trust the dogs together, but now I am ok with it. I was a bit nervous about the GSD hurting the smaller dog. I kept the GSD on a leash at first. They are never alone together either. We crate both when we are gone.