Our original two cats are rescued ferals (part of a farm colony) that we only got when they were a year and a half old. They were quite set in their ways but adapted to indoor life very well. Neither were afraid to use teeth or claws if they felt threatened but it didn't take them long to realise which side their bread was buttered and soon behaved as if they'd been born to indoor life and servants and both became very cuddly cats.
We took in two foster cats a year later and our original male, who was fairly snappy when we got him, immediately took over and helped them to settle in while his mate threw a swipe every chance she got, as if she resented them being there.
Shortly after they left we adopted two kittens and the same thing happened. He played the father figure and she dealt out the slaps and didn't want to know. The kittens are now over a year old and our original male is the one they go to for games, for cuddles and for washing while an uneasy truce exists between them and our girl. She doesn't appear to like any other cats except her mate but she's at the stage where she tolerates the kittens as long as they leave her alone.
In the interim, we have has a few short-term foster cats and he's always the one we take in to see how the new arrivals react to other cats and, even with a hissy cat, it doesn't take him long to get around them while she will sit outside the door and hiss at them. It's quite comical to watch.
The kittens, despite being just over a year old, are eternal kittens and everything's a game for them so we try and keep them away from any short-term guests if possible - at least for now.
We will, no doubt, have more foster guests in the future but it's always a process of trial and error as to who gets on and who doesn't. You just have to make sure that your own cats know that they're still the most important people in the house and they will work it out between them.