I agree with PrettyKitty. Cats can get along... over time. When I rescued Gracie, she and Jester did not get along. Gracie was bigger at the time and tried to make herself the dominant cat. A few months went by and Jester out-grew Gracie (now she's over twice her size) and they all have their place in the hierarchy chain. We also have a third cat, Princess, which is Jester's mom. In the end though, I make sure that I AM the alpha-cat. When a fight breaks out, a loud and quick "HEY" usually does the trick, but sometimes I have to hiss at them (something I learnt as a cat tip) and once in a while I have to "bite" them (hold them by the scruff of the neck like a mom does to her kittens). My best advice is to get some books/DVDs on cat behaviours. I got a video once and it helped me understand their interactions much better... like the hissing. A cat doesn't understand us like a dog does; they're more into body language. By hissing at a cat, you're telling them that YOU are the alpha-cat. You can try separating them, but not locked away (closed doors) because they need to know that they have to put up with each other sooner or later. Oh yeah, one point. Since the male was rescued, it will take him longer to adjust to the new environment. Our rescued cats took over a year, almost two, to fully come around.
Good luck!!
Cats can eventually learn to tolerate one another. It make take several months but they get it. My grandma had a case like yours. It took about 9 months for the cats to tolerate each other. My husband's yellow cat was slowly introduced to a white kitten and he'd hate her with all his strength. In the end, and after a year, he just ignored her and let her be. But, your cats can get along. They can!!
Patience. If a fight gets out of control, separate them or make a loud noise. Good luck with whatever you decide to do!
You've got a serious problem on your hands here! I personally think that some cats will never be able to get along with each other, some tolerate each other, and others get along great. They are just like us!! From the sound of it, it sure sounds like they can't tolerate each other. That's going to make for a very stressful life for your whole family.
Honestly, I am not even sure how to go about trying to fix the situation-you can't make them tolerate each other. The only suggestion that I can make to you is to separate them as much as possible-ESPECIALLY when no one is home! I am seriously concerned for your female kitty's safety. Maybe someone else has got some other ideas, because I sure don't. I'm sorry, sure wish that I did!
Best of luck!! Please keep me posted!