Single target -- yes. Additionally, if you get a thundercloud proc and a Firestarter proc at the same time, your instant thunder spell takes priority over your free Fire III spell. The reason for this is that Thundercloud has a chance to immediately proc again, while firestarter does not.

AOE. I would suggest getting accustomed to hardcasting the first flare in this rotation and using swiftcast for the second. The reason for that is it's more likely your target is nearing death after your first flare and it really sucks to waste a global cooldown by blowing your second flare on a target that dies before you complete your cast. But this rotation as you describe is fine. Just pay attention to your targets' health and switch targets as necessary.