Well, this morning, Caramella (the other, slightly larger, shyer, foster girl)
was wanting to see in Mavis' beddroom, so I opened the door. Out Mavis runs.
Mavis bristled, and was just inches from Caramella. Mella proceeded to hit Mavis on the head repeatedly. It didn't deter Mavis.
So they raced and chased around the foyer, and postured and smacked eachother. And Mella even would persue Mavis, eventually the glimpses I caught were of Mavis always chasing Mella. So, I ended the play session.
I think it was a good step.
I didn't think Mella would hold up so well, as she still has minor issues with the 'big cats'.

Hopeful.