The back alleys of our neighborhood are filled with strays and ferals, with new crops of kittens nearly year round. The end result is that we have four adults and two kittens in our household now. They all get along with one major exception: a few years ago we took in a young cat (about 10mos) and she absolutely hates our oldest male cat.
We think he may have bullied her when she was living in the alley behind our house. He's a bit of wuss, not bold enough to pick a fight with another adult male, but he runs off the adolescent kittens who get too close to our yard. But Willow is not a kitten anymore and she holds a grudge. Big time.
They can't be in the same room together, so we're always shuffling them into different rooms and closing doors so she can't get at him. The very sight of him through a window or through a glass-paned door is enough to send her into a fury, hackles raised, growling. And he's terrified of her now. He won't come in the house until he's sure she's not lying in wait for him. She even stalks him outdoors, but he's quite athletic and she's rather puffy and slow, so he gets away easily. Good thing or she'd have chased him into another state by now.