Tactical Map setup plus other question

Not outside of changing equipment and choosing character class. Troops with ranged weapons go to the back, troops with melee weapons go to the front. Spellcasters (Mages and Commanders, mostly) tend to go to the middle if given a melee weapon, rather than to the front. I suspect the defense and dodge scores also have an influence here, hence why Mages and Commanders, who typically have low defense and dodge, go to the middle rather than the front when armed with melee weapons, though the game may also be checking traits.

Against spells? No.  Against fire or cold or lightning staves? I believe so, as dodge works against these weapons and I don't see any reason why ranged dodge would not.