well.. it's more that you can either have one, or introduce multiples all at once..
Adding more down the line will lead to aggression between the established ones and the newcomer.
As with any group that's added, I imagine there will be a short stint of pecking order establishment, but if they are healthy and properly acclimated and fed, shouldn't lead to a death.
(according to the specimen's specs that is...they all have their own personalities, can get a bully from time to time).
I also think they are aggressive to any tang, not just their own type...not sure tho, best to wait for the advice of a tang-a-holic I spose