I'd strongly advise against a sixline wrasse.
There are two issues with these fish, firstly, though they stay small they are very active fish which like a decent amount of swimming space and enough room to form a territory. I keep one in a 24G cube and would say that it's probably a little too small for it, although I will be moving it to a larger tank in a couple of months. The second is that they can be very agressive towards other fish, not always enough to hurt the other fish, but certainly enough to stress a new addition enough to put them in serious danger. This is only going to be worse the smaller a tank gets.