Seeking "cat-whisperer" advice ---- RE: New Cat in the house, compatibility with my older cat, etc

[Warpy]

8. One thing I did was close the resident cat in the bedroom

when she was snoozing on the bed and let the invader cat out to explore the rest of the place, rubbing his scent on things, exploring the other litter box so he knew where it was, checking out the furniture for sleeping suitability. When I'd hear the resident cat scratching at the door, the invader cat would be back in his own area. The resident cat would then start sniffing all the things he'd rubbed against. In that way, they'd start getting used to the idea that it was shared territory.

It still took me a broom, a blanket, and a glass of water at the first F2F introduction of the last two adults I had to introduce. After that, they de escalated, just hissing and swiping and generally being cats until I caught them sleeping together when I came home from work one day. Fakers.

Some adults bond pretty quickly. It took my last two some time but if those two can bond, any cats can. Both had been solitary cats in their first homes.