[Taqwus]

Light of the Northern Star description in the manual and in game explicitly indicates boost to all astral mages, not just friendly. Cheap astral mages without getting lucky on randoms aren't too likely until you take a lizardman province.

As for keeping them out of battle... you'd also have to keep them in a fortress, to prevent Teleport/PotS/Duelx3/Returning or similar uglineness. Even cheap Star Children would be a problem; they're much cheaper than a CM, if memory serves, and are S1 assassins, and you can't stop assassins with a fort.