I am with lankeela - I think you are being very sensible in researching the breed BEFORE you get one. I was a member of my dogs breed club for about 4 years before I was in a position to buy my dog (I had 2 GSD's and a BSD at the time and could not take on a fourth giant dog). It does not mean that you love your present dog any less.
When I eventually got my pup, my GSD was old and in a K9 cart with CDRM. For the last 12 months of her life she brought him up totally and she adored him - she always did like puppies. She did not lack any attention from us and he just added to her quality of life.
One thing I would say about Russian Black Terriers is that they are a working/guarding dog and a large one at that. I found this video on you tube (not very good quality) it gives you an idea of what they can do.
https://www.youtube.com/watch?v=5bN1im1BT1c
There has been at least one in the UK that was a police dog. Don't forget they are a Terrier and can be stubborn and are large enough when adult to get their own way!