Q:
Rune of the Flame Legion question.
Your primary stat as a direct damage engi your primary stat is crit damage. That being said ruby orbs are the cheap/easy way to add so e extra crit damage and of course crit and power.
Flame legion is an intersting choice, it would be some extra power and 5% damage while you have burn up. With incendiary powder I could see them being viable but overall still less effective than ruby orbs or scholars.
My advice, pick up some ruby orbs you will be glad you did.