They've been fully exposed to each other for over a week now but my little old lady (she's like 10 but nearly kitten sized) Nyx is just absolutely fucking terrified of the new cat. I haven't seen them fight, but to her credit the new cat does sometimes charge up to her quickly in either a "I wanna play" or "i'm gonna eat you" mode (but idk which because Nyx runs and hides immediately. I don't THINK she is charging up to fight, though.)

Part of the problem is, tbh, I feel like Nyx has some kind of anxiety condition? She has a high degree of skittishness that she's never really gotten over. Lots of fear responses to things (i.e. the sound of a trash bag prompts her to run and hide). And I can just tell looking at her she's 100% fear response when the new cat is around, i can see all the gears in her head just churning "i'm afraid im afraid" over and over

We were maybe going to try ordering one of those calming pheromone products to see if that might help but idk, i've never used one before so idk how effective it might be

After typing all this out though i'm wondering if she might need like, a cat SSRI, since she has a lot of anxiety in general unrelated to the new cat

I was thinking maybe it would be a good idea to just keep them both in one room all day, monitoring them to prevent them fighting, so that they have to get used to each other? but i could also just see that scaring Nyx all day without her learning that she can relax, because she hasn't become less anxious to her other triggers through exposure

P.s. the other old cat Mary Jane, is much more fine with the situation. She hisses some but they've gotten a lot closer and more smelling/grooming attempts without real fighting