What do you do when your pets do not like each other?

Been there many times with cats. Most of my personal pets have been acquired one at a time as adults, so there is always a period of adjustment. The first one took two days. The next one took twelve years. The next one, he had two months of smelling and hearing her in another room, but when she was let out hissed at her for three months until he got it through his thick head that she was no threat to him. When he died, she took a dislike to the new female cat I brought in, and who also took a dislike to her. They both wanted different things, and they both wanted those things to themselves. After a month or so of cross words, they agreed to ignore each other. Then I took in a male who wanted to play with the girls, who did NOT want to play with him! So he bonded immediatly with a former feral male I took in, and they're such good friends that I can't bear to split them up. All in all, It takes time and patience, but if they're not actually attacking each other I just let them work it out. One of my girls is now getting intolerant of sharing with other cats, and I have her isolated by herself. I'm thinking of finding her another home where she can be an only cat because she will be happier that way