I thought that our old cat Hollie would NEVER get along with the new kitten we brought home just about two weeks ago. I mean, Hollie used to attack stuffed animals. I figured she would NEVER get along with a REAL cat and I was afraid that she would hurt the kitten. Anyway, we brought the kitten home and confinded her to her own room. Hollie would sit next to the door and hiss at the paws she could see under the door. I thought it would never be good. We did eveything on the post about introducing new cats to old but I was heartbroken to have to leave this little 4 month old kitten crying by herself at night. So only three DAYS after we brought the kitten home we decided to let them see each other. I held the kitten in my lap in the doorway of the room she was staying in while Hollie sat just outside of it. I sat there until I could no longer hear Hollie growling. After that I brought the kitten out to the living room and began playing with her. Hollie would sit in a corner and wait until the kitten was not looking and CHARGE at her and hiss and spit and growl and raise her paws like she was going to hurt her but she NEVER did. In doing that though, the kitten learned that Hollie was the alpha cat of the house. After that day, they have been fine.
My advice to you is to try holding the new cat where the old cat can see it, and see that its not as big of a threat as it thinks it is. I also think you should let them take some swipes at each other. As long as there is no blood you should be fine. If one of your cats wanted to hurt the other, they would do it. If that happens and the fur is flying, then you have to take the new cat away and confine him until the old cat settles, and try again the next day. I think many times, cats need to work it out by themselves and they need to establish a hierarchy and right now since they haven't really seen each other for a long period of time, there is none and that makes them both nervous. I am by FAR no expert, but I did manage to get a cat who hated even stuffed animals OK with a kitten in 3 days