Where did you get the idea of an Eldar Pirate Exodite? The idea behind the concept is that it's an auxilliary in a Tau army, usually a young Eldar like a Ranger who believes that the Tau will be the galaxies only hope once the Eldar are gone and so have allied with them to help them as much as they can.
It is, however, absured. The oldest race and the youngest allying? The Eldar are much to proud for that. Let alone to adopt the Tau's symbols and weaponry in place of their own.