I'm trying to make the http://download.fliggerty.com/download-4-1063 work alongside psy's http://mw.modhistory.com/download-56-11850, so his unique dremora models would use the new armor. How should I go about doing this in nifskope? Or do I need to use another program?

Not sure about this, but since Psy got his head models from a Playable Dremora race mod, I think you could implement your combined critter as an NPC, and add those heads to it and give it the armors from the armor mod. It would be a simple way to get the look of what you want. You could script it to ensure players couldn't rob the armors off the corpse.

Anyway, that would be what I would try doing first, because I'm completely ignorant on how to model using nifskope/blender.

"Morrowind Comes Alive" implements a couple of the dremora companions in this fashion.