If you get a sixline, make sure it is the last fish you get. Then I think you will be OK. Mine tangled with both a royal gramma and also attacks my cleaner shrimp, and will go after the Nassarius snails if they come up out of the sand. he does not even look at any of the corals, the Astraea snails or the feather dusters. The wrasse and gramma get along great now but the wrasse really tore the gramma up when I first put the poor thing in there.
If you add one last, though, you'll be happy, I think, they are very fun little fish with lots of personality. You just can't ever let him think he's king of the tank!