I did see your post yesterday and meant to answer but had an emergency and had to log off.

In my experience, after a while, most of the cats I had living together, even if they did not love each other as such coexisted peacefully. I even had to take my sister's two adult male cats (and my sister ) to live with us for a while. In the beginning there was trouble with Gigio (RB) who was 16 and the only male for a long time. My sister's cats were younger and much stronger and wanted to become the dominant ones. After a while they started tolerating each other ( I had 5 of them 3 males and 2 females). When in September I brought a kitten home one of my females would not have anything to do with him and would attack him. It lasted a while but now they are often found together.

I would try to give extra attention to the older cat, sometimes it's so easy to get distracted with a kitten since they're so cute and cats can be very jaelous too. It takes time and patience but , in most cases, it can be done. At least it worked for me.

Again, sorry if I did not answer you before. I hope I was at least of some help.