How do I get my 5yr old cat to accept a not quit one year old cat?

I have a spoiled rotten 5 year old cat and have rescued a not quite one year old cat from under the dumpster at work. I made the mistake when I brought the little one home last night of just setting down the carrier and expecting my older one to start acting nice! Didn't work!! Now I'm trying to follow the advice of the experts and have them separated in different rooms. I'm curious if anyone else has tried this method and what the results were. My older cat has been labeled the "devil cat" and such by my friends..so that tells you what I'm dealing with. The reason I want another cat - besides the fact that I love cats - is that my older one is getting fat and seems to be getting crankier the more I work - guess I figured that she'd enjoy some company and stimulation. Please give me any pointers or anything that you've learned!!!!