I thought you were looking at another gsd? Maybe I misunderstood?
As for another breed that would be calm,,if your going the rescue route, which is a good idea, most older puppies/adults in reputable rescues are pretty well screened, so that would be a big help in locating one that you feel would do well with your gsd.
Not necessarily a "breed" as it would be an individual dog. I have australian shepherds as well as gsd's..and let me tell ya, they are as different as night and day..My female aussie is very demanding/high energy, where the male out of working lines, is much more "go with the flow"..Yet he is not real good with dogs he doesn't know, she loves everybody..I'd never stick her with a dog who needs "calmness" LOL..
Ok done rambling,,find a reputable rescue and go from there if that's your desire,,keep in mind herding dogs (other than the gsd) can be really high maintenance..
Diane