I don't think the size of the tank is too small as much as that we are dealing with an aggressive damsel that is well established in the tank. In a case like that a much larger tank would be too small.
The only damsels I would even consider are clowns or a chromis.
To answer the original question, it's possible the six line is still alive but hiding from the damsel. Six lines eat pods which can be found in rock work or at night. I'd try removing the damsel, since you won't be able to add any new fish anyway, and see if the six line eventually reappears.