Hey @manorvillemike like I said to you long ago and as you are experiencing.. its no easy task, I mean even today some of mine like to start a fight.
Like what @deerjay said.. cats are just like people some will never get along because they all have different personalities.
So here is the advice having a metal cage to put Mary in with the other cats could indeed be a great introduction, next I am afraid is the worst part you need to bring Mary into the room and watch what happens take note of who starts the fights, also make sure Mary as a place to go in the room so she can be alone, cats love having their "own" place in the house some of mine for example used to fight but once they had a place it stopped the fights, also I have used "telling off" cats who start the fight, point your finger and say no! You need to do it a few times and some will eventually get the hint that its bad and you wont be happy and hopefully behave, it takes alot of patience my friend and sometimes along time. Please keep us updated on Steemit and we will try and help, even if it means saving up enough Steempower for a bigger house lol.