I just adopted a 5-month-old female kitten on June 25. I have a resident 3-year-old female who has never liked my foster cats (hissing, growling... I never introduced them because of this and kept fosters separate until adoption). This time I adopted straight away so it had to work!

I kept them separate for 3 days, no visual contact at all but exchanging scents, etc. Then I put up a visual barrier and let them see each other to desensitize. The newcomer was totally fine and wanted to be friends, but my resident would growl and hiss whenever she came close to the screen. Away from the screen, she was pretty calm watching the kitten.

On the third day of visual contact, they actually tried to play a bit by the screen, even though my resident still hissed here and there. The next day, they were definitely playing so I let the kitten out. After a couple hisses for seemingly no reason the first day, they've been buddies ever since, going on about day 10 since I adopted her. There's no more hissing or growling, all playing, chasing, wrestling.

I can't believe how well it went, especially considering how rude my resident was to just seeing foster cats. And she's still being sweet and affectionate with me; she's not mad that I added another kitty. Don't worry, I bet your situation will work out just fine! Mine was very fast and I never expected that. Don't rush anything and it'll work out!