I see level -1 mercs all the time. When you hire them, they're the same as mercs with no experience.
Also, the "Zoroastrian Male 0" bug happens with all Confucian wedding events. No idea why. It might affect Taoist wedding events, too (since Taoism also comes after Zoroastrianism in the religions), but it's so rare that anyone has Taoism as their state religion.