You wouldn't know it to look at my six line wrasse, because he looks peaceful and what not ... but he's a killer.
Proceed with caution is all I can say. They are cool fish, but can have mean streak develop. Not as bad as other fish mind you, but, enough to be wary of this fish.
My own observations are that a pecking order is very important to this fish. Anyone who can do more damage to the sixline, than the sixline can do in return, will be respected and not harrassed. Anyone lower on the totem pole, however, i.e., those who the sixline can do more damage to them, are the ones who will be occasionally harrassed. Occasionally that harrassment can be an all and all out assault (which the victim may not survive).