The shelter that I volunteer with always recommends a seperation period of 1 week where the new cat is confined to a floor or a room and the other cat(s) are encouraged to sniff under the door and get used to the smells. Then open up the whole house to both cats and let the new one investigate. Keep an eye on your other cat as she may need to assert dominance. Let them growl and hiss and chase, but if they get to rough then seperate for another 2 days and try again. There may be some hair flying before they settle in, but most cats will decide their pecking order without fighting. Springing them both together can cause problems with your existing cat as she may feel her domain is threatened. They will learn to tolerate each other. Good Luck.
Kim