1) Yes, the order is preserved no matter what throughout the whole game. The only complication is if they're coming from different provinces there is no way for you to tell what the order will be - but it will always be consistent if you paid attention earlier.
2) Entirely situational. Fire magic's highlights are flaming arrows, fire brands, shields of gleaming gold, lightless lanterns, incinerate, falling fires, and several really nice situational things like flambeauxs and rune smashers. You only need to go into fire if you need to go into fire.
