Communicants are an exception in more ways than one. They take less fatigue than theurgs from casting spells in communion, especially for high-path magics. For instance, in a test game with an S5F4 caster, 2 communicants took a total of 112 fatigue (8 + 108) when the master was scripted to <Communion Master, Astral Shield, Flame Storm>. 2 theurgs scripted to <Communion Slave> took 23 fatigue from casting Communion Slave, 31 and 41 more from Astral Shield (making 54 and 64 respectively), and 115 and 136+ from Flame Storm (making 181 and 200, and the 200 guy was down 1 hit point). I have no good explanation for this, but Pythium communicants are far better than normal communion slaves for fatigue. Especially if you do like Wish and give them Standards of the Damned so they can Drain Life.
-Max