why do people keep saying that?
501st DO NOT stipulate the medium you use to build your costume, the costume just has to meet the Accuracy requirements and CRL requirements.
Its not what you build it from or how you build it, it just has to be accurate, they do suggest getting one of the available kits, but they don't stop you building your own.
there are troopers in the legion with FG/ABS/HiPS suits
You have the Mando'a creed in you signature, so I assume you are familair with TDH and the Mando merc's, some of those guys have built their Mando/Fett kit based on pep and been approved.
I am presently working on the Sith Stalker armour (from pep) and have been told as long as its accuarate they will add it to my legion costumes.