Also... I'd like to add about males introduced to females...

My son adopted an orange tom 7 yrs old from someone who had to give him up (moving).

His other two are females. The middle one Tima, gets into a "sexual" thing with him. She came into heat twice before he could spay her (before Garfield showed up).. So she has some female behaviors set.(she stole into his house when he had the door ajar in November weather at his old apartment when he went to throw his trash in the dumpster). She had decided to live there since she saw other cats in the window, and so he kept her. She was very thin, but healthy, and lived a hard life in dumpsters. So she interacts with Garfield differently that Ellie does (who was spayed earlier). Some play with cats involves growling and some screaming. It is a personal trait and different for each animal. If the ears are not flattened, but there are vocalizations, then the interactions are not very alarming. Aggression involves lowering the ears for all cats. Some females just "warn" off males, because that is how they interact. Tima will growl at Garfield sometimes, and not others. We did not see one vocalization at Xmas and they all got along well. Even when the food was given!

In general males will "keep trying" as that is ingrained in them, and we watch Houdini slowly close the space between him and Oreo. It took him 2 weeks to get Weezie to play at all, and yesterday we saw him sleeping with her on a cat tree ledge! First time in MONTHS. He wants to cuddle desperately. But he is slowly making progress with her! She was afraid of him too, but slowly is accepting him. I don't know how they will be after he is neutered yet...that will be a month more I think.

I think slowly the fearful cats will relax, especially when they are "treated" in a relaxed state. Withhold the treats, when hissy. Don't react to hissy either. They may interpret your tension as a confirmation of their suspicions! Cats are very sensitive I've learned. Many mistakes I've made myself. And in the end some are extremely stubborn...like Sheba was.