I wonder if the solution is to adopt two cats together - maybe kittens around the 5 or 6 month mark. They would be company for each other, and less likely to bother Bella.
I think we were very fortunate that when we lost Ross and got Moray, Moray's character was such that he actually spent time and craft persuading Tinks that he wasnt a monster, and was actually quite delightful. The fact that he then truned into a teeny terrorist is neither here nor there - by then, his work was done, and Tinks was hooked.
It also helped I thnk that Tinks is still a bairn himself - 18 months old.
I agree though - worth checking what they can tell you about Bella's interaction with other cats before making a move.