Some cats are friendly to other cats and some want to be in a single cat household. Their personalities like ours, differ and who they like or not is a matter for their own taste, so there's not a whole lot a human can do about that part of the equation.
In time the cats will sort themselves out and get along as best they can. I think its important to give them equal attention, if thats possible.
I read somewhere too, at the beginning of the introductory period; rub your finger along the side of the new cat's mouth and then wipe your finger along the side of old cats mouth. About 3 or 4 times a day till the first cat accepts the new one's scent.
A I write this I hear howls from one of my cats - the big neutered Tom just has to keep bullying the rest, the others don't like his bullying ways and neither do I, even though I love him.