Cleon
Legend
I'm hoping you meant 10-21 HD (Gargantuan); 22-27 (Colossal). If so, I agree. If not, let's do battle!
Added to Homebrews.
I'd meet you halfway with 10-13 HD (Huge); 14-21 HD (Gargantuan); 22-27 (Colossal).
Thoughts on the poison suggestion upthread?
Well the venom is only an ingredient in the truth potion, it could be just as lethal as regular wyvern venom and cause disorientation/delirium.
Maybe add Wisdom damage?
Poison (Ex): Injury, Fortitude DC 20, initial and secondary damage 2d6 Con and 2d6 Wis. The save DC is Constitution-based.
Improved Critical (bite) or Improved Multiattack for its extra feat?