Join a Pommy board, I think they exist? Course they all might be clueless. You GSD "is" top dog you have to make sure he knows that.
I only knew one Pommy, and he was top dog, two dog family the second dog was submissive to the point of embarrassment! But AFAK, Pommies always think they are top dog! That is what I would be on the look out for.
Leerburg On Demand | Introducing a New Dog into a Home with other Dogs
If you get a different breed, do your research on the new breed, I didn't my GSD took exception to my BullMastiff/Pitt mix Dominate Male... GSD Dominant Aggressive Male!!! My only "advantage" was that they never really hurt each other while "I' was getting my act together!
A toy dogie can't handle many owner mistakes!