I have 3 males of various ages living with me. The last boy I brought in, Nino was not a cat friendly cat but I had to have him
He's a huge silver tabby Maine Coon. Anyway's I kept him seperate for about a week, then started leaving the door to his area open a bit and just let him come out and poke around and then I'd put him back after he'd had a bit of out time. Now the resident two boys were a bit stressed cause he tended to be a bit on the aggressive side, chasing and smacking and screaming!! So I discovered calming collars, they have a pheromone and lavender in them. So I bought one for everybody and they really worked well. Took the chasing and stuff down to a manageable level and everyone was able to chill out. I can't remember exactly what they're called but you can get them in Petsmart. They're purple. So fast forward a few years and two years ago I rescued a female kitten, she spent the first little while seperated until her vet visit and an all clear. Now I think because she's female, everyone accepted her pretty quickly and other than going into heat...wow!! (never again!!) That and being a kitten, they put up with alot from her. So I would try the collars for both the kitten and your cat.