Any way you can keep them apart longer and let them get used to the fact that there is another cat in the house by smell only? When I introduced Scout I kept her in the guest room for about a week. I let her out to explore the house on day three but I put the other cats in the sunroom where they could watch her through glass doors. On day 5 I let them meet for a little bit - hissing, growling and a few smacks. By day 8 I would let Scout out all the time I was home but put her back in the guest room when I left and at night. By day 10 I felt comfortable enough to leave her out full time.

Going slowly is the best way to make sure they have time to not feel threatened. Meeting too soon can get them off to a bad start.