My experience is exactly opposite of steve's. I had 3 large ones that I added all at once. They did fine...untill I dropped a rock on one and a brittle star got the other one. For about 5 months I just had one big chromis. Very cool addition, but about a month ago SWF had them on sale for 4.99. I added 4 of them to the mix. These things are so small I can lay the entire thing (fins and all) on a quarter. The large one never pays any attention to them. They will generally school...but not always.
The "odd # rule" really only applies up to 5. Beyond that, there is so many they dont notice. Its not like they can count.