Adding them all at once or within a very short time period is going to be preferable to adding them incrementally.It's not ideal for most people to go out and buy every single fish in one go.
I'm just saying that it's fine adding more along the way.
While it may not be an option for some people it is better to do it this way if possible
Whenever you are dealing with African cichlids it is a matter of probability. There is no absolute things that will work or not work. There are, however, things you can do to increase your chances for success and and this is one of those things.