Okay, here's a question:
If you send out a Vampire while Nightfall is out, then it would have 4 HP, so it could survive a RoF. But what if the Nightfall is destroyed afterwards? Does the Vampire die, since now it would have 0 HP?
EDIT: Also, why can't mutants take the form of Devonian Dragons or Immortals? Other dragons and immaterial creatures are allowed (ie. Phase Dragons), so why not those two?