I've heard you are supposed to introduce a cat by keeping it separate in a room for a day and they first get used to the scent and do a little dance under the gap under the door ...
That came from a crazy cat lady, lady at the rescue place ... so, YMMV ...
That's how I've always introduced my cats when I have other pets, although I keep them separate for more like a week rather than just a day. First with a closed door then with a baby gate between. Otherwise they end up quite stressed and hissy.