I feel your pain! My late tortie hated the rescued kitten (Beo) whom we adopted almost thirteen years ago (both have recently passed on, and were at last, to the end, inseparable). However, they did start out on the worst foot possible (with the tortie, then about three years old, spraying around the house and attempting to attack the new little boy through/under/around and over just about every door/gate/blockade we could set up). We did for a time have the little boy living in our walkout basement and Elle(the adult) living upstairs, and it took them a good year of antagonism through the basement door before Elle would tolerate him sitting across the living room from her (while she gave frequent warning growls). I'm not sure of the dynamics/behaviors between your current cats and the new addition (Beo was a very good natured baby who always wanted to approach Elle, who seemed to take this as a sign of aggression), I do think many of their problems were because we didn't at first enforce an extremely gradual introduction process. They did eventually get along (after a year), when Elle finally accepted/recognized that he was just a socially inept baby (even at 1-2 years), and felt confident that she was ruling the roost. However, I really do think we could have reached this point a bit quicker had we taken more precautions from the start (or, had we had the sense to request a redo, as you have :)). Not knowing what you've tried the first time around, I can only offer some unspecific suggestions; one of the best books ever written on the subject is Pam Johnson-Bennett's Cat vs. Cat- my sister raises Ragdolls, and told me to read this book even before I was considering a new cat. She gives step by step instructions for introducing a new cat. A brief summary (hers is much more detailed) follows.
First, YES! Initiate operation Introduction Redo! Pretend the past days/weeks/months never happened. And then progress as though the new cat just walked in the door (it's never too late to take a few steps back before you step forward again).
So, use the safe room for as long as necessary (cats are not, contrary to popular belief, naturally solitary animals- they're a social, colony species that needs time to establish the hierarchy with a new cat- it may seem impossible at first, but it will happen eventually). When your current cats show interest in the safe room (which they always do), distract them with some interactive play (my cats have all loved Da Bird) to draw them away from the door. Be sure you maintain the regular schedule for your current cats- don't give them more treats or any more attention than usual (I've always found that many people recommend the opposite of this, but Bennett makes what I think is a very convincing argument against it: cats thrive on order and regularity- your altered behavior will not "reassure them that they aren't losing your attention" but will instead confirm for them that something big and potentially life-changing is occurring). After a few days/possibly weeks (when your cats stop attempting to claw your new cat under the door/seem less distressed by the safe room) do the Sock Exchange (it can be a Small Scrap of Fabric Exchange I'm sure, but be sure the piece of cloth is small, so you don't overwhelm the kitties). Rub the piece up cloth along the chin/jaw of the new cat, and place let your current cats smell it (if you get an aggressive response to this, give them some time, and then try again. If they seem curious, rather than hissing/aggressive, then leave the sock somewhere where your cats spend a great deal of time (their cat tree/your bedroom/etc.). Next, you will need to do this same exchange with your new cat (if you have multiple current cats, you will need a sock for each), repeat this process with your new cat smelling the other fabrics/socks (the same rules apply: if he reacts badly, slow down, if not, place those in his room). You might repeat this every other or day or so, until the cats seem to accept the new cats smells. Once they do (again, this may take some time), move on to the Room Exchange.
For this, you may need another person to take the new kitty out of the room for you (if you have another room in your house that you can place the new cat in temporarily, do that. Otherwise, have a friend/relative take the new cat in a carrier to another room). Then, open the door to the safe room, and let your current cats venture in as they're comfortable. Let them explore it, familiarizing themselves with the new cats smells (this room graduates from the sock, which was minimal and contained, and now they must become accustomed to a room where your new cat has left his scent everywhere from rubbing around the walls/furniture/etc.). Again, watch their reactions, how comfortable they are, let them explore until they're bored with the new space.
Let your new cat get similar exposure to a contained area used by your other cats (don't overwhelm him with the run of their house immediately). Watch for his responses.
If all goes well, progress to a sight introduction (have a friend/relative on hand to help keep the cats physically apart). Do this from across the room if possible (my sister recommends starting out simply holding the new cat where your other cats can see but not reach). Again, be watching for their reactions, are they weary, growling, curious? As always, any signs of anxiety mean you should take a step back (another sock exchange- perhaps a few more from the new cat to keep the scent in different places around the house), for a few days, then back to the sight introduction.
Once you've moved through these steps and the established cats are showing less signs of aggression, you can move on to letting the new cat physically meet the group. As always, watch that they go through the appropriate social niceties without reacting aggressively (nose sniffing, tail sniffing, etc.)- frequently a rambunctious young cat might skip one or more of these and discomfit the older cats by jumping right in as though he already belongs- make sure the older cats can comfortably inspect him without his attempting to initiate play time too quickly.
As always, some squabbling will occur when a new cat is introduced into an established colony- their hierarchy is already set, a newcomer throws things off (who gets the highest seat on the tree now? who sleeps in the master bedroom? who gets first aykies on the food/water?)- this time is stressful for all of them, and it may take them a while, even after the cat has been accepted, to determine the reordering of their hierarchy.
Elle and Beo were, after a year of squabbling (which was largely caused by good natured antagonism on Beo's part) finally established that Elle was, of course, the queen of the house, and Beo was the acknowledged clown. Even at 12 and 13 he would still have those times where he would try to bait her into a minor tussle, but they were very closely bonded within some two years of meeting. When Elle died in her sleep less than a year ago, Beo eventually stopped eating, and no number of trips to the vet, tempting treats or syringe feedings would work to bring him around- he died only two months after she did. My point is, I suppose, that cats are truly social animals- your new cat may at worst only be tolerated by the others (but they will find a place for him eventually), or he may end up closely bonded to one or more of them. Given time though, they will gradually determine how to make room for the new guy, and peace will reign in your house once again :)
