1. I don't like the way ghosts attack
2. a) it doesn't look right
b) how they deal damage while inside (wouldn't it be simpler to just kill the creature immediately)
3. I probably wouldn't change the attack (since the concept is good:) ), but I would add an ability that would kill at least one creature - this will create balance issues, and to solve them, you'd have to slightly lower the ghosts' stats.
Awaiting your assessment...