Auto-cast preference?

Is there a way to set the preferred targets for an autocast ability? For instance, I have an ability based off of Faerie Fire, but it does a different effect depending on if it is cast on an ally or enemy (with dummy abilities, obviously). Is there a way to change it so that it will autocast on frendlies even though Faerie Fire is a debuff? I was thinking about having 2 abilites, one based off of Frost Armor and one off of Faerie Fire, but I don't want to do that if it's avoidable...or is there a better ability that I can use as the base ability? Thanks in advance!

You can set it so that the ability is targetable on friendlies if its originally intended to target enemies, but I am not sure if autocast abilities will also be casted that way. Best thing you can do is use a disabled spellbook with the auto-cast Frost Armor in it. Then you will add it or remove it when the faerie fire based ability's autocast is activated or deactivated.

Ok, so the question now becomes, is there a way to make the ability only castable on units considered "ethereal"? I know I could just make a trigger that, when the ability is cast on a unit not considered ethereal, tells the casting unit to stop and displays an error message saying that it must be cast on an ethereal unit. However, since this is an autocast ability, it would be quite annoying to the player if the ability was on autocast because the hero would constantly be being told to stop. Is there a way to rectify this shy of making the ability not auto-cast?