It's more of a 'turf war' I think. I went through the same thing with a cat, Lucy. She had always been an only cat. When she was 5, I got Leroy. I had never heard a cat growl in their throat the way she did. She wasn't growling at him but me. She was so mad. They eventually got so they would tolerate each other but were never buddies. I went through the same thing when I got Luke and Chester. It wasn't Lucy this time, she had gone to the Rainbow bridge. Leonardo is the ruler of the roost now. He doesn't let the others forget it either. He still swats the others from time to time just to let them know he is boss. He especially likes to stalk Chester because he is the youngest. They are getting along better now that Chester is 2. Chester never fights back, he just walks away from Leonardo. Leroy, Luke, and Chester all get along fine, it's just Leonardo who tries to be a bully.
I guess to sum it up, they will probably learn to tolerate each other but may never be 'best buds', but that will be ok. Good luck.