A red mage ninja has already been done. the Ninja from FF1 is basically like a red mage. I mean, he's red, uses awesome ninja weapons, and he uses low level magic! If that's not a Ninja Red Mage, I don't know what is.

A Red Mage Pirate however, is going to be much more powerful than a RMNinja. However, the RMNinja could probably sneak the bejesus out of the RMPirate. But if the RMPirate spotted the RMNinja, it would simply be a matter of who pulls off the dirtiest tricks. They'll both be trying, but only a true master of the unclean can win.

I prefer RMPirates, but the RMNinja would more likely win.