I have heard it helps to add multiple wrasses at once, that way one does not get a chance to set up a territory.
With that said, I did not have a problem adding by blue sided fairy wrasse to my tank, which had an existing six line.
And my six line must be the exception ... he does not pick on the other fish. He will hold his ground when the stupid damsels try to bully him, but he has been model tank citizen. The only time I have seen him get aggressive is when he sees his reflection in glass.