We've had a breakthrough at our house, where we've been trying to integrate two young Scottish Folds into our household of three rescued strays. The Folds are here indefinitely until their companion comes home from the hospital, and at a year and a half old, they hadn't seen other cats since they were small kittens.
It was clear initially that our cats, averaging about 10 years old, were not interested in adding more felines into the mix.
At first we separated them in the upstairs/downstairs fashion, and they could just barely see each other through a gap between the door and the wall. I noticed plenty of curious glances and door stakeouts, so after a week and a half I started carrying our cats, one at a time up the stairs.
There were a few puffed tails and a hiss or two, but all in all it went well. No one leaped out of my arms into any kind of fray. So I recently took the plunge and opened the door -- and voila! A five-cat household. Not one fight. Some territory has been staked, along with the afore-mentioned hisses and puffed tails. But one of the Folds especially has taken to our cats, even coming downstairs just to play with them and join in their dinnertime.
It can be done!