Can Primaris Marines mix elements of older patterns with Mk X Power Armour?
Practically? Of course. Primaris models are made so that heads and shoulderpads (at least) from Firstborns can easily fit. Gravis models accept Terminator shoulderpads.
However. Is there justification in the lore for doing so? Does the Codex Astartes or the revised practices introduced by the returned Roboute Guilliman cover such a use of Astartes equipment?
Given the restrictions i have given myself for this project, i wouldn’t want to build a model that explicitly goes against established lore in novels or rule books.
We know there is some incompatibility between Firstborn and Primaris equipment because they can’t board each other’s vehicles, at least on the Tabletop.
So, can Primaris Marines Armour be complemented by elements of older patterns, similar to how Firstborn Marines in MK VIII armour can use Beakie or Mk III helmets and shoulder pads?
We need to be sure.
Thanks to a comment on a post on r/40kLore i got the tip that this topic might be covered in Aaron Dembski-Bowden’s Spear of the Emperor novel.
With this novel confirming inter-operability, i can proceed forward with planning the Amnis Transitores armour additions and trims.
As mentioned in Kitbash Notes, my demi-company of rubicon marines will have relics and armour elements of their past life as Firstborns, so it’s nice to have established lore backing the practice of mixing and matching armour patterns even across the Primaris rubicon boundary.