First thing to do is get him neutered!
That will decrease his desire to go out, and make him more pleasant to live with. (Even though your cat is spayed, if the new guy still has all his equipment, he will be motivated to impress her by spraying his manly scent on every vertical object he can find! I can't imagine that he hasn't been spraying already.).
Once "manly" cats can become "pussycats" once they lose the motivation to fight (cats with balls live primarily to mate, so anything else they do--eating, fighting, spraying--is all because of that biological imperative). I once had an outdoor cat neutered, and after that, he was always around my house and never strayed (I couldn't take him in at the time, but he lived in a warm crawl-space under the house, and I got him his shots etc. He had come from the outside, so was street smart, and pretty much became my "outdoor" cat).
After that, I've found that male and female cats, or two males, get along better than two females, so that's one plus.
I have gone through several "new introductions," and currently have 5 cats. Every cat is different, of course.
My Simon has never hissed or growled at another cat, and is quite copacetic. So is the newest guy, Rascal.
Scooter is jealous as heck of new cats (he's a "mama's boy"), and does the "I will rip you to shreds" growl/howl when a new introduction is made (there is never any physical violence, though, just the threat). He also gives the new ones the "evil eye" when I pay attention to them. But he eventually comes around (about 2 weeks until total calm and acceptance). When he smells outside cats through the screen or screen door, he yowls and sounds very scary!
Mandy is the least cat-loving of my cats. She still doesn't want to hang out with the others much, and is not really friendly to them, but just avoids them. If she's on the bed with me, and someone else jumps up and gets too close to her, she leaves. I call her "attack cat" (I even have a sign!) because she has at times whapped at the other cats.
Bobsie is a very sweet, motherly type, but she's not too fond of Rascal (he's HUGE and she's tiny). She now accepts him, but doesn't wash his head and head bump with him in the morning like she does with Scooter and Simon. At first, she grumbled and growled at him, but now doesn't.
Most cats will hiss and growl at new cats. I'm always amazed that two complete stranger dogs can meet, and seem to become instant friends. Definitely NOT a cat trait!
I volunteer at a shelter, and they have three rooms for cats, with no cages. There can be as many as 14 cats in the adult room. They don't all get along, but they tolerate each other (except for a few "cat hatin'" cats, who would probably avoid the others if they could).
All this to say, it IS doable. It just may be a while befoe things get calm.
When you first introduce the two cats to each other (make sure new guy isn't contagious first, though your cat probably has a better immune system than he does and could fight off the illness faster).
Then, get one of those baby gates that you can put in a doorway, withh new guy in one room and your cat having free reign of the rest of the house. They can smell each other (and spit and hiss and growl!) as much as they want through the gate without doing harm to each other. (My guess is that the guy will be more peaceful than your female. These things work best if at least one of the cats is more peaceful. Even bully cats don't seem to have fun for long beating up on a nice guy!).
You can get them used to each others scent (which is probably their most important way of identifying each other--I feel sorry for new cats introduced into the cat rooms at the shelter: even if they stay far away from the others, on a shelf, they are still suddenly surrounded by the odor of more than a dozen new cats, which must sometimes be overwhelming). Take a dry washcloth, and rub it over cat A's cheeks, chin, and temples (where there are scent glands), then introduce cat B to the scent by letting him/her smell it, and even rubbing it on him/her. Do the opposite as well.
Make sure to give each cat a separate bowl (just common sense, IMO, but some people make their cats share bowls, which they don't necessarily like. My 5 each have their own bowls, and they pretty much have learned to eat just at that bowl---though once in a while, they seem to decide to play "musical bowls"!)
Give your kitty lots of attention when introducing the new guy. Even when you are petting him, use her name and talk about her, and glance at her. (You might see her giving him the "evil eye" the way my Scooter does--funny, because when he does, I talk to him, and he looks at me with a "what? what did I do?" look!). Make sure that the introduction of the new cat doesn't change her life too much. Ie: if she's used to sleeping in the bed, make sure that she can still do it.
(Hmm...is there a word limit on posts here? YOu can tell a post is long when your internet connection breaks of because of "lack of activity". LOL!)
Hope this helps (and is not TMI!).
