I've never seen any particular recommendation, just senior kibble. I can't even remember right now what kibble I used for Razzle when she had falsies..thank God she's spayed now!

I think the important part of using a senior kibble might be a lower protein content, so definitely try something other than TOTW, where most of the varieties are pretty high protein.
I feel your pain about prednisone..Razzle has been on it for over 2 years now for her liver disease. The long term side effects are...not fun. But if it keeps them going with decent quality of life, you do what you gotta do.
Just for general informational purposes about false pregnancies for everyone-if the bitch decides a stuffy toy or any other object is a "baby"-take it away from them! They'll just worry themselves sick over it, all you do is prolong the agony if you let them keep it.