ANSWER - It's rare I answer a question on here with an "I don't know." Quite often I have to research an answer and sometimes I chat things over with other witches but usually I manage to offer something better than a confession that I'm truly boggled...
But this time I have to confess that I'm stumped. And the reason I am so certain that I'm stumped is because I am in exactly the same position as you are. We have four established adult cats and a very young cat who decided to come and live with us last year (no luck trying to find owners). One of our older cats hates the newcomer - hisses at her, spits and tries to assert her dominance at every opportunity. Lots of aggression and nastiness between them and to date nothing magical or mundane that I have done has made any difference.
Calming the cats via magical methods hasn't worked, nor has keeping them apart at night, water spraying them when seeing bad behaviour or making every attempt to reassure them that they are loved. The only thing I haven't tried that I have been recommended and intend to try myself is using feline pheromones to promote a more relaxed relationship between them. Link here
I'm sorry I can't be more help - I want the answer to this one as much as you do!!
What I will do is put this on the Facebook wall with a shout for advice and see if anyone in the community has any ideas that might help you...
Good luck! (And if you get there first - pass it on x)
Image http://www.freeimages.com/photo/1344085%20 iggypcat