So.... it might help Carrow to be a little tougher to offset his poor/not heavy armor?
Carrow isn't wearing any armor! Well, he's wearing an Elf Helm, has an Elf Shield, and is wearing an Elf Cloak. But he didn't actually buy or find any armor that suitable to him so far. He could hang onto the rune for now and buy some Elven Armor or hope to find some other kind of armor.
Can Ty use it? Because of his armor restrictions adding Toughness wouldn't be a bad idea...
Ty could indeed use it, but keep in mind that his armor covers sections that correspond to the enemy's to-hit roll. His body armor works if the attack that hit him rolled a 6 to-hit. If he later buys other pieces of pit armor, he can get leg armor corresponding to 5, right arm armor for 4, left arm armor for 3, and a helmet for 2 (most enemies wouldn't successfully hit him at all if they rolled a 2, but that isn't always the case). If he puts the rune on his body armor, it
only applies to his body armor. This would be one of the few ways that Ty could boost his armor above 1 point, so it's not like it doesn't benefit him, but he's also the only warrior for whom the effect is partial. So it's a bit of a tradeoff. The party probably wants Ty on the front lines and anything that can improve his defenses is good, but the boost on another character would apply 100% of the time that armor works, while in Ty's case it probably applies to fewer than half of the hits that he takes. That's individual hits. For damage, it works out a bit better for Ty because some monsters roll more for damage if their to-hit roll is 5+. Not sure how to weigh all that, but I guess my impression is that it's a bit of a tossup. It would work, but there'd be a drawback.