I took in a kitten from the outside once when it was storming. I already had a tuxedo cat that was a year older (the dr thinks) and apparently the outside cat was in heat. I left her in a spare room for a couple weeks, where I let her sleep in one of the older cats beds. After that I switched up there beds for a week so they could get use to each other's scents for like another month (they would paw at each other under the doorway). After that I opened the door and let them interact. They've been best buds for 10 years now.
What would you guys do?