ORA also sells mated pairs of their A. Ocellaris.
Sounds like a waste of money, unless you're in a hurry. If you buy two juveniles and wait, you will end up with a pair.
Speaking of which, my ocellaris just spawned for the first time yesterday, woo-hoo! They have been courting forever, but never found a spot to their liking until I rearranged the rockwork to give them a big cave. I guess that's why real breeders use flowerpots. Maybe some day I'll have the time and energy to raise them. They'll be plankton this time around.
JoeyD, nitrate will go up after the nitrifying bacteria get established and if you have no live rock to get rid of it. Stress-zyme won't do anything at this point (although I've never used it), and you may as well stop adding it. The only way to reduce nitrate is through water changes.
With regard to adding new fish, I would add a small, rather than a big fish. If your fish is already differentiating as a female, somebody will die if you add a bigger fish. Tomatoes are agggressive enough as it is. How big is the tank? Those guys need plenty of room.