A few months ago, I was on here looking for advice regarding adopting a 3 year old Australian Shepherd that turned out to be very much a "one person dog", the type that tried to bite my DD twice, bit my DH once, kept going at my 3 year old female GSD, and grabbed one of our cats and gave it the death shake (and it only dropped the cat when I held out a treat) all just to keep anyone from coming near me, the only person it attached to.
When the dogs met at the shelter, there were a couple of very subtle incidents that did make me wonder if this was a good fit, but the shelter people all reassured my family that this dog came from a home with two other dogs, it loved dogs and cats, was great with people, etc., etc. All it needed was time to get used to us and our dog. I suppose they were only going on what they had been told when the owners surrendered the dog, so I should not fault them.
The concensus here, for I did come here for advice, was that it was in the best interest of my GSD that was already in the family to return this new dog to the shelter. I even had my present dog's in-home dog trainer come to my home to try to help, but she also said to return the dog. Though it broke my heart, we did take him back. In hindsight, I only wish I had never even brought the dog home.
We tried for 9 days, and in that 9 days, the tension level in my home had risen so much, that it was like a breath of quiet air hit us all in the face the morning after the dog went back. My Shepherd girl was not shaking, her teeth weren't chattering, she wasn't looking around the corner in fear, the cats weren't cowering, etc. I am so, so sorry that we let down the other dog, but the advice here of what was best for my present dog was truly spot-on.
Pea-n-me, you mentioned that your present GSD is a bit "different". Mine is also, bad start in life, no socialization, scared of everything (but has improved with training and love, and is very different than when she came here!) but she really, really enjoys playing with her stable temperament dog buddies. She does not do well with any type of aggressive or dominant dog, and we came to the conclusion that we just could not go on what a few minutes meeting at a shelter would become when it turned into days in our home.
Plus, we were apparently black-listed from all the local shelters because we returned the dog
. We did try to adopt again, but no shelter would even call us after any applications were sent.
Knowing how much our dog does like having a playmate, and the fact that her Maine Coon buddy is not going to be allowed outside to run with her, we adopted an 8 week old pure-bred German Shepherd male puppy. Best thing we ever did!
Our girl is so happy, adores her little "brother", and he adores her. They play, occasionally have a little tussle, but she isn't afraid of him, which is wonderful for us. He is 13 weeks now, and weighs 42.8 pounds, so he will be huge (she is small, only 52 pounds), but I know they will be best buddies until the end.
I am glad I put my girl's comfort over my desire to save that other dog, and I do wish that we had been able to save a shelter dog, but it was not meant to be. Perhaps, if we had been able to adopt a younger shelter dog, the outcome may have been different. I suppose, when you have a resident dog at home that does need a little extra consideration, it is harder to find the right fit.
I wish you good luck, Pea-n-me, in finding a friend for your Shepherd. I do believe you made a good choice in not taking the lap-dog, and in taking into consideration your present dog's comfort level. I know how hard it is to want to help another dog and not follow your heart, but rather following what your brain is telling you. Your present dog is lucky to have owners that want was is truly best for her!
p.s. - regarding wanting a lap dog, my little boy GSD, the one that is 42.8 pounds at 13 weeks, he desires to be a lap dog. Not sure how I feel about a dog that will eventually be over 100 pounds being a lap dog, but he sure is darn cute when he asks to cuddle in, unfortunately for him, we don't allow him on the sofa, but I don't think he will need the sofa to get in our laps, as he will be very huge very soon!
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BUT I love her to death she is now my baby and sits on my lap all the time and sleep with me. I think if I give my lab attention she does get jealous and has to be right there but she hasn't bit him FOR that anyway. She is a true ankle bitter and will chase him around the house biting his ankles. SO I think it is possible just depends on the dog.
After that, she became reactive to other dogs, and most times reaction is fear-based. It took many years to teach her that nothing bad was going to happen when she saw another dog (which is why it was frustrating for other dogs to come after her). 
adding they see some tough ones in there and she herself was a little hesitant about Shepherds anyway. (I'd explained ahead of time she could be reactive so I think she was a little nervous about it!) That was part of what made me feel like she (and I) deserve some peace in our lives at this point. 
