I don't see any reason why Toughness is a prereq. The PrC is basically a cleric class that gains a new Domain, Alertness, a Wyrmling, Sleep & Paralysis Immunity, Darkvision & Low Light Vision, and can Summon a Dragon. So I figured changing the Toughness prereq wouldn't be a big deal since it doesn't seem to add to the flavor of the class in any way.