I like the idea of a cap-only unit that has a non-fire fire attack, it would definitely help them a lot. Perhaps even make it sacred, I'm sure that as a race of pureflamium(tm) they would revere those whose flames burn the hottest and brightest
You could maybe even get away with making it non-mr resistable, just an ap attack with no fire effect. Or maybe an AN single target attack with high precision? And everything you said in three makes sense to me also.
Yes, fireball is aoe piercing. The reason I suggest no miscellaneous is because it would be really hard to give them equipment that wouldn't just melt, thematically speaking. But if the commanders had decent stats, casting something like body ethereal, personal luck wouldn't make for a bad thug, especially if it had a native fire shield. Though they might have fatigue problems....