It's something really simple in the OC module, something like "npc helmets=no" to "=yes". Or if it's in the player.bic of every save, I don't remember. Enabling helmets was a common procedure a few years ago, but now I can't find any trace of it on the net! Doesn't anyone here know it?
And yes too, the helmets fit. They are off by a few inches on some of the companions, and you got some slight clipping or hair collisions at a few of them, but especially Khelgar and Neeshka benefit greatly from certain model headwear IMO.
One of my old screenshots, showing Shandra Jerro with a very matching circlet:

This was from March 2010.
Khelgar is a monk here and doesn't have anything on in the pic, but in my current playthrough I will let him remain a warrior, plate helms look great on him.
Grateful for info!