Don't give up, it will work out in the end. A few months after one of my cats passed away we found a kitten and brought her home. I already had a 15 year old male (also a rescue cat). For the first few days there was a lot of hissing but then things calmed down. Shortly after my sister came to stay with me for a while with her two very strong 3 year old cats. One of these and my old cat hated each other on site but after few fights they decided to ignore each other. When they went another young female joined the family and, in the beginning, she just did not get on with my male cat (who is now 18) and now, when noone is looking, they go to sleep on the same bed!