Highly likely he wasn't expecting that :P. And yes baby Gen does have a tail. He actually has it up until after he's gone through a bit of his magic training. Though he technically didn't know Shizu in his early years, age regression potions being left in the wrong places sometimes produce such situations much to their guardians' displeasure.

In their world: These potions reverse all effects on body that happened after specific age reverted to and put a sort of mental lock on a lot of knowledge that would otherwise be potentially mentally harmful to children of the specific age. Recognition of others they know however may or may not be changed. (EX. When Renshu was accidently reverted he lost his fear of lady parts. Memories of what happened after that age were locked away and the fear along with it. He still recognized Shizu, Khan, the Chief and Gen.) Yay for magic worlds that make little sense :D