Fire, Flamestrike, Fire, Flamestrike, Fire. Why?
Flamestrike is Alternating physical and magic builds the chain gauge faster.

it starts tossing Watera and Aero into the chain. Just to make the fights look prettier.
ra spells do bonus damage to staggered enemies (Fira / Watera / ETC; they do about as much as two fires, then bonus if the mob's staggered) and Aero it's the games way of increasing the chances of juggling them succesfully. Also, If an enemy is resistant to spells, it will puts in the elemental physicals only. If and enemy is resistant to physicals it puts in spells only (usually).