new cat is hyper-aggressive with old cats

First of all, sorry for the long post!

My fiance and I took in a 10-month old male stray a couple of months ago, and at the beginning, he got along pretty well with our existing cats - a 4 year-old fat orange male and a 3 year-old tiny siamese female. We probably introduced them much too soon (within a week), but they got on quite well at the beginning (some hissing, but mostly they ignored one another). Unfortuneately, since then relations have been steadily deteriorating.

First the new cat started bullying our big male, who is generally a easy-going wimp, so it wasn't really surprising that his dominance would be challenged. Despite her small size (she's only 6.5 lbs), our female has always been a toughy, so we were surprised when the new guy started going after her a couple of weeks ago. Now, every time the new cat sees her (which isn't often, as we now keep them separated) she bolts, he chases her, and if he catches her, he attacks her really viciously. Today he managed to escape from his room when I was going in to feed him, and he caught my female and attacked her so ferociously that I got pretty torn up tearing them apart. I honestly think that if I hadn't intervened, he would have killed her. (I'm not kidding, I've never seen cat fights so one-sided and savage before.)

Now, we're hoping to give the new guy (who is super sweet and playful with humans) to a close friend and her husband. The problem is, they have to await approval from their condo board, which seems to be taking forever and is not a sure thing. So, if we end up having to keep this guy for longer, is there any way to reintroduce him to our cats and undo the damage? We'd happily keep the new cat (he really is a gem, aside from the anti-cat aggression) if he could get along with our existing cats.

A few more details:
- We had the new cat neutered about 3 weeks ago. This didn't change his behaviour at all.
- I've successfully introduced cats several times before, with no problems (which is probably why I was overconfident and introduced this new guy too soon this time). My original two cats usually get along well with other cats (they stay at my parents' house with their 2 cats when we go away).
- I've heard Feliway can help, but haven't had any luck finding it here (Calgary, Canada).
- The new cat is VERY affectionate to people, but occasionally play-aggressive and aggressive from over-stimulation.
- I'm probably not helping by yelling like a crazy person whenever he starts chasing my female cat, but it's hard stay calm and rational when my sweet little female is in mortal danger!

Any suggestions would be very much appreciated!

Yup, all three cats are fixed.
Thanks for your reply--I really hope he does calm down in a couple of months. I'll keep my fingers crossed!

It's good to see that I am not the only one going through this right now! This site has been great in helping me deal with these issues, that I have never faced before with previous pets. My new kitten who I adopted at 5 months as a stray from a shelter keeps doing the same thing to my 5 year old very sweet female calico. He attacks her non-stop, but I found that he's slowing learing the word "no". I also walk around with a squirt gun that I have to use when he attacks her. I know it's because he was a stray and she's a female who may or may not be in heat (she was a stray too but I got her from a friend who wasn't sure if she was ever fixed or not). The squirt gun has helped calm his attacks by 50% or more. He's totally loveable most of the time, she just drives him crazy whenever he see's her. I heard from people who have adopted strays that it takes about a year for them to really adjust to your home and new life. In the meantime, I would try getting a squirt gun. My little one loves water and doesn't mind being squited, but now he's affiliating the squirt gun with "no". When he sees it pointed at him, he now chooses (most of the time) to hop over the calico and attack something else.

Originally Posted by michellem

. My new kitten... I heard from people who have adopted strays that it takes about a year for them to really adjust to your home and new life. In the meantime, I would try get a squirt gun.

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A kitten is naturally playful and curious. Are you sure you are not confusing his playfulness for attacks? It could just be play fighting.
A squirt gun is useless. A firm NO sometimes followed by a hiss is enough.
Finally, the time for adopted strays to get used to their new home depends mainly on their age and history. It may take as quickly as 1 day or more than a year.