Best way to get them more friendly..... FOOD.... great food treats for each one.

Every time the others are not showing anti-social behaviors, give them attention, tell them they are good kitties and give a favorite tidbit...(here it is Chicken or shrimp). A special treat at least once a day for each one at a different time also goes a long way.

Oreo is finally accepting Houdini... he is very cautious around her. And she has forgiven me somewhat for taking her to the vet for the needles! (and medication stress). I sit and talk to her quietly with a whisper, telling her Houdini wants to be her friend etc, and it is working. But I tell ya FOOD wins every time.(at least for us). Oreo weighs 8 lbs too and is 13 and has cancer. She looks huge and weighs 8 lbs... go figure!

Reward the new kitty each time he is non-aggressive too.
Fresh catnip goes a long way here too!

Try and keep very cool/calm when the hissing starts... they feed off you and your tension. I used this calming when I was taking Oreo to all those Vet visits... she'd cry and tremble. And I'd lull her and it worked, slowly, but it worked. If they sense you tense they feed off that.
I'd come home exhausted, though... as it took alot out of ME.
She is sitting with me now, here in the computer room and when I was doing some music for us... she decided she liked some of it too. So now she comes to me in the afternoon (hoping that day will be special tuna day), and falls asleep to the new music.
Here is an example:
http://www.youtube.com/watch?v=5L7px9EicFs
I discovered this music from a cat video where a cute cat did Yoga with her owner to this musician. Some of his stuff is louder than others. But Oreo likes this one alot.