I had an old cat and a young cat and like you, I felt the older one was being bullied and getting a bit stressed by it all. I ended up getting another kitten and it worked perfectly. The 2 youngsters became best pals, playing and fighting together, while my old cat got peace and quiet and cuddles. She would snuggle up with one of the others when they were asleep so she still liked the company but she didn't get all the grief she'd been getting before. She passed away a few months ago and the added bebefit of having 2 youngsters is that they had each other once Shelley was gone.