I don't know about adding that many, because I don't know their bio load, and max size, but I know part of hat is true.
It is better to add them at the same time. If you had some in your tank and then added just one, it would get singled out and picked on, because it doesn't have an established territory. You also want a lot of drift wood and hiding places, plants, rocks or anything to keep them from seeing each other all the time.
I hope this helps, and I hope someone else that knows more about them will comment.