I can't remember but one instance where we absolutely failed in getting two cats to at least tolerate one another, and that was an adult unneutered maie that was really territorial being brought into a house that already had a cat that thought he was tops.
What's worked for me, personally, is to put the new cat in one room of the house with their own litter box and all for a few days. The cats can smell one another, but not fight. And after a few days, I'd bring the new cat out to explore the house and put the other cats in the room the new cat had been occupied for an hour or so to sniff around.
And then eventually they meet face to face and there's usually a certain amount of posturing, and in the case of Halvah and Cleo, they were never friends...but they did get along. And that was an adult on adult deal. Usually when it's been kittens, they learn to get along pretty well.
Well, I worry that 'Cartman' could really do damage if he decided not to tolerate them...
Kitty #6 tormented Ramius's Josie all the years he was at the beach house, but never really hurt her. He just hassled her.
I wouldn't have thought it would work to bring another cat in with Cartman, but he really did pretty well with that lost kitten, and we just threw them together rather unexpectedly because she just showed up.