Why not make the Gheallie Sidhe a special training option rather than a special elven unit?
My thoughts: Looking at two elves standing around, you cannot say if one is Gheallie Sidhe or Neut. (At least not until he opens his mouth... )
And if the Gheallie Sidhe is meant to be more of a violent hit and run unit, if would seem to me that having that option for Archers or Cavalry might be a good idea.

The Gheallie Sidhe could be a special training option like Scouting or Toughness available only to Elves. This way, it could be applied to any Elven unit rather than it being based on a Knight or Cavalry unit as is popular. And it would allow a GM to storyline rogue Gheallie Sidhe units as commanders quietly train their troops in this option. (Useable against a pro-human elven ruler(My troops are being trained in WHAT?)- or useable by an elven elitest ruler against those meddlesome breeding humans... )

I would suggest a cost of 1GB, +2 melee +2 missile +2 morale vs humans, must pillage if in a human province.

Or perhaps it could give a different bonus to reflect its use of guerilla warfare?