I've introduced all my cats one at a time as well, although they were kittens which makes it much easier.

I want so bad to let Boo-Boo out of the room. He really wants to come out and when I hold the door open just a tiny bit so he can get a look at whichever cat is in the hallway, he whines and reaches his paw through the opening. (I do it for the safety of MY cats because I'm still not sure if Boo is cat aggressive or just scared and overwhelmed). I might try again this weekend to let him out with maybe one of my cats. But then again I wonder if I'm just wishing it'll work out when I know in my heart he needs to be the only kitty.