Here you'll find rules for making dragonhide armor. I figure its the closest thing in core 3.5 to making armor out of enemies. The Tarrasque is Colossal, which is four steps larger than Medium, so I imagine that you could make at most 4 suits of masterwork Tarrasquehide armor. Additionally, it can make 1 suit of masterwork half-plate or full-plate Tarrasquehide armor.
2. im not saying having ALL of those qualities, but considering that you have killed an innately magically and unique creature, armor-made from its hide should be fairly magical and unique as well. Maybe instead of the immunities, give it minor resistances, similar to the magical enhancements to armor.