One of the pygmy wrasses is your only real option in a tank that small. If you're unable to source those, maybe look at some other smaller active fish as possible alternatives. Maybe look at the group of pseudochromis fish as a possible wrasse-like fish that could work in there. Maybe the orchid dottyback would be a good one. You could also look at something like one of the assessors, but those tend to be more cryptic in my experience.