I have read that if you spray a bit of perfume on them (all the same kind), they find out that they all smell the same and don't fight anymore. I have never tried it myself. Another thing I have tried and it seemed to work was petting the new cat and the old cat at the same time, as if to say, "see? Mommy likes them, why can't you??" I sometimes wonder if the resident cat feels the need to protect it's territory/people from the new ones.

Hope it all works out for you and your family!