You can put the extra slot in any ToT weapon, fyi.
At any rate, yes, Gao-rei is better with the 500 agi gem. Weapon DPS doesn't help our survivability much, while agility really does.
---------- Post added 2013-04-15 at 11:25 AM ----------
While it's called 'Rot Armor', it actually just increases the damage we take by a percentage. It has nothing to do with armor, thus there's no preferences for who tanks what.
I'd actually say we should go on the red head, since we have more cooldowns to utterly negate or at least reduce the breaths significantly (Glyphed Guard, Diffuse Magic, Zen Med, Fortifying Brew).