and welcome to TCS............youÂ´ve joined us with an interesting question. I can relate my experience when I introduced a kitten to a 2 cat household
My existing 2 cats (Dino & Pepsi) were a little younger than yours (both about 10/11ish) but they had been together since kitten-hood, so I had the same worries as you.
I did the slow introduction, fed my newbie in her own room for about 2 weeks, then I started leaving her door open, the others would want to have a nosey and I monitored it very closely. She had her own cat-lit box and toys in the room.
If my newbie (Moofi) wanted a race around, I would put my others in another room whilst she had the run of the house. They didnt mind and usually fell asleep anyway
In time I moved Moofis feeding bowl closer and closer to my other two and that worked fairly well. They all realised they would get fed, so no battles ensued.
A couple of times she would want to play-fight, there was the odd spat or two, but generally cats do settle down to whose who and how to behave.
I totally agree that the slow introductions with monitoring was what worked with my crew ! It does take time and patience but its worth it when they are all buddies
and keep us posted...................oh and some photos would be great when you get chance