[Gregstrom]

I like this idea. Sharkskin armour for the tribesmen sounds right, and I have the vague idea that it might have been used historically.

For the headhunters, how about an undead secondform? Kill them once and they become an ghoul-type undead of some sort, but don't turn back after the battle. If you give headhunter leaders a tiny amount of undead leadership, then the undead won't flee the fight but will have to stay in the province after the battle is over, unless a shaman is on hand to take charge of them.

This might be a step too far, but... maybe make the undead form quite strong (good hp, str, att, and 2 claw attacks), but have it eat population (1/month or some other low number). Then you have this unit that's good for its price, but that you don't want to have in a garrison.