Aggressive new cat

Question
Dear Tabbi,
  We have rescued many cats, and had successful introductions many times, but our latest resident is so unpredictable that we're looking for advice.  Riley is 2 -3 years old, and he showed up on our porch.  He was un-neutered, and obviously had been outside for a long time, but was VERY friendly to us.  We had him neutered 8 weeks ago, and have been keeping him separate from our other cats, but are now trying to introduce him.  We have 4 quite mellow cats that are housed together in a 700 square foot area, 2 males, 2 females, and they all get along very well. (They were all adults when they were introduced to each other, and we never had a spat)  We first put Riley in a pet taxi, and let everyone sniff him.  He purred and kneaded the entire time.  After a few sessions like that, we have been letting him out for short periods while we were watching, and after 30 min or so, we  put him back in his own room.   While with them, he pretty much ignores the other cats, and trots around happily.  The problem we're having is that he will be perfectly happy, show no signs of aggression, even play side by side with the other cats, then suddenly attack. It's not play fighting, he's serious.  He mainly goes for our sweet female that is rather timid.  Last night everyone enjoyed their play time.  Then Nala was resting, with her back to him, and he suddenly hissed, lunged at her, and came away with a mouth full of fur.  She never fights back, she just hides.  At first I thought it might be mating behavior, but 8 weeks seems like a long time, and the hormones should be settling down.  He also sometimes goes after the boys.  The other day he walked around giving everyone the stare-down, whipping his tail.  When these signs of aggression occur, we stay calm, and gentle remove him from the room.  None of the other cats have been aggressive to him, and the 2 males keep touching noses with him and trying to be friendly. But we're afraid that if he continues to go after them, we'll have even bigger behavior problems.  We make their time together as pleasant as possible, but then he goes off without warning.  We love this little guy, and are trying desperately to make this work.  
Thanks, Nancy

Answer

Dog Crate for Cats Nancy,

Sometimes there are cats who behave like that because of their personalities and they won't change. I would try letting him back out on the porch where he was happiest. Sometimes penning up a cat that has been free will make them aggressive. Also a feral cat that has scrapped for his food will still retain that behavior, and also he may need his own 'space' and territory, and have to share with other cats.

Hormones usually take about a month to exit the cat's body, but sometimes the neutered cat will still act like a tomcat 2-3 months later.

Once the cats can be together without fighting or freaking out feed the cats yummy cat treats periodically (like tuna, tinned sardines (in oil) or pieces of raw steak, etc.) TOGETHER. That is so the cats will associate each other with something pleasant. Also getting a pet laser light (at PetSmart, Petco, etc.) is great for interaction between the cats. They can chase the 'little red bug' on the floor, ceilings, and walls, and they tend to forget about the other cats as they focus on the 'bug' which helps them get used to each other (and you).

There is a calming product that you can add to the cat's food or water that reduces anxiety called Bach's Rescue Remedy. It is available on-line and in health food stores. Here is a link about it: http://www.bachflower.com/Pets.htm

I use a big dog crate for a new cat. It has room for a litterbox, food and water, small cat carrier without the door for a bed, to hide in, and to get up on, and they still have room. That way they can get used to other cat(s) around, and other cat(s) can sniff and check them out safely, and a new cat is not locked up by itself in a room which can be more frightening and confusing for it. You can gradually let one cat/kitten out for short periods of time as they start getting used to each other, and switch cats to let the new kitten explore. I got mine at Petco. I am attaching a picture of it.

Tabbi