I would be very cautious in doing this. I added a flasher wrasse to my 120 with an established six line and the six line killed it. He chased it relentlessly for days on end, attacking it when he could. The rest of the time he would not allow it to come out and eat. The flasher was twice the size of the sixline and the sixline hadn't bothered anything else in the tank. It's a gamble and if you're prepared to tear your tank apart removing one of them, then go for it. Obviously, your mileage may vary but there are many reports of sixlines doing damage to newly added wrasses. A large fairy wrasse might be a safer bet. Flashers are more timid and don't stand up well to that sort of aggression.