Socializing two cats

There's my resident female cat. She's 12 years old , almost 13. Shy, talkative, insecure. Sweet personality but when confronted with a strange cat will hiss and spit in fear.

Then there's the new cat. He's male , 4 years old. Came from a family that had him for 4 years and then he was put in a shelter. The shelter did not have him in a cage the whole time. He was actually in a house where there were 60 other cats. At night he was put in his cage but many other cats roamed freely around his cage at night. He was in this situation for 2 months. I'm guessing this is what's made him aggressive.

I've been doing good socializing the two. Kept him in a separate room the first week. Now it's been 3 weeks. I thought things were going good. Just this week end, I had them playing a bit together (playing with string, no actual contact between the two), and they touched noses a couple of times in a friendly manner. I was feeling very positive. Then last night he startled her , she spat at him , and then he started chasing her!!! I caught him and put him in his room. My female cat was very shaken. She spent the night on my pillow looking for comfort. This morning she threw up her meal (maybe stress? I might take her to the vet). This isn't the first time this has happened. He seems to go along fine most of the time (95% of the time) and then he will become very aggressive toward her. Each time this happens it's a major set back because my female cat becomes scared of him and I don't want her to be frightened in her own home. Any ideas? I've been trying non-aggressive ways of trying to reprimand him. (like just putting him in his room) and I thought this was working but it appears not. Has anyone else gone through this situation? I don't want to give up on this cat but if he's going to make my resisdent cat's life a living hell , then I might have to find him a new home. I hope not. He's quite a sweetie and he doesn't need more change in his life.

Thank's for taking this shelter-cat in to your home:thumbs up
I have 3 males(10,8 and 3 yrs old),who get along great,but there are occasions,where rough play gets out of hand,then my little waterspray comes in handy:D
One of mine will never accept my youngest cat and he hisses and sometimes spits,should my young one get too close,but never any serious cat-fights.
Some aggression at times is normal,if they annoy each other in some way.
Your situation is a little different,since it really seem to bother your girl and I am sure she is stressed,but please don't give up yet.
Since the aggression does not happen all the time,chances are they will eventually accept each other,things might turn around and your girl will be the boss.
You seem to have done all the right things and that they played together is a good sign,it takes time,lot's of love and patience.Good Luck!

As long as there is no sustained aggression (hisses are fine, she's just setting limits) things should work out. They may not be best pals but they will find their own level of tolerance. If the male is not neutered, have it done immediately, it will help settle him down. Never force them to interact, play with them separately, with the male to drain off some of that energy and get him used to you, and the female to reassure her that she's still your sweetie. They should have their own beds (or a blankie), and a space where they can retreat in times of stress. For now, I would also feed them separately and put them in different rooms when you are away.

Thanks Chico2. I'm trying my best. I guess 3 weeks isn't that long afterall and I'm pretty proud of my girl for being as brave as she is.

Thanks Badger. Yes, I'm doing all the things you mentioned. They each have their own eating area. He sleeps in his room at night and she has the place to herself but she usually sleeps in my bedroom. During the day she has her safe place to sleep too, in the office on the desk. He's was already neutered by his previous owner's so there's nothing to do there. As for their playtime I didn't think I was forcing them to play. I'd take a long string and have him play at one end and her at the other. Or I'd play with one while the other watched and vice versa. They both seemed fine with it. In fact she was the one that intiated a play session once.

And yes, they are being kept separate when I am out. I wouldn't dare just leave the two alone when I'm not there.

I am glad you are not giving up on the new guy,so very often we have read"He's aggressive got to get rid of him!"
Yours are already making progress,seeing they can be together in play:thumbs up
I too kept the new kitty in his own room when I went out and for feeding,until one day my older cats just gave in and realized the"new"guy was here to stay:D
Let us know how they are progressing:fingerscr

Thanks Chico2. He's such a sweet little kitty and it seems we are finally starting to bond so I definitely want him to stay around forever. I was just concerned that he might never stop being aggresive to my female cat. However, it seems it just takes TIME. Lots of time. I'll just have to be patient. I will definitely let you know in follow ups how it goes.