Any advice on introducing a new cat in the home?

[TheKitten]

Hi.

One my neighbors recently left, and she had a whole bunch of kittens. (didn't neuter... grrrr!) that ended up being abandoned. One of my neighbors took one of the cats in. And I've also brought one inside. That one is a bit older (young male - not neutered yet; will take care of it ASAP).

I have one male and one female already, both neutered.
My male seems to have accepted the newcomer quite well. He just ignores him.
But my female is having a bit more trouble. She spat at him a couple of times, and the new cat takes a submissive attitude. She also smelled him. So far, she's just shown an aggressive posture towards him, but hasn't actually hit him or anything.

I think they'll be all right, but I was just wondering if anybody had any advice on making the transition smoother.