[thejeff]

If a caster casts it without a communion (or a ridiculously high Astral level) he'll max out at 200 fatigue.

I believe the matrix casts at the beginning of the turn. Regardless, it is in effect when the mage casts.
I would just use a matrix for the master and have him act after the slaves, thus getting the full effect of the communion and being able to cast first turn. The slaves would cast Communion Slave normally. (Pythium communicants would also work). Then follow up with Vortex or Antimagic to prevent reprisals.

slave matrices should be treated like any other item when it's bearer dies. Commanders on the winning side have a chance to pick them up.