Mohawk supposed to wiggle because it's supposed to break off in a real fight. It's just decorative rank symbolism, if it was solid and didn't break off, if it got hit would prolly break the wearer's neck.. same reason why visors and stuff detach on motorcycle helmets, so incase a branch snags it it doesn't snap your neck back , I assume it's similar if someone hits a solid fin on top your helmet lol, that would crunch your neck whichever way
Your head is too big for Greek helmet. But, maybe if you put some padding, than it'll fit better. Like medieval knights had cloth padding to soften impacts from heavy blows such as maces. Try some padding in between