I doubt they'd even need to use all 6. Ice and Water for example have no real merit. Black Mage uses ice to recharge and Hyoton is crap in PvE, no one else uses Ice beyond that. The only Water attack I know of off hand at least is Suiton, which is mainly used to proc Trick Attack. Earth is used mostly by White Mage with Red Mage using it on Verstone procs and Ninja using it for Doton, so Earth can afford to be ignored here. That leaves Fire, Thunder, and Wind as the most reasonable resistance debuff additions.