[thejeff]

Grudge: I don't think you can do what you want, though the answer to your actual question is "yes".

If he's powerful enough to cast Fire Storm without the communion, he could cast it, then cast Communion Slave, but with multiple masters and only 1 slave he'd die very quickly.
If I understand correctly, you want the pretender to act as master for 1 round to cast Fire storm, then become a slave. That can't be done.
You could have the pretender as master to boost him enough to cast firestorm and then have him not cast, which would let the other mages cast, though he wouldn't absorb their fatigue. Getting him to not cast is slightly tricky. You could retreat him. If he's thug material he could attack, or you could give him a bow and order him to
fire.
Either way, I'd probably have him cast PotS first. +1 to all the slaves paths is probably worth a round of casting.