My armor is Styrene and ABS, and is one of the very few I know of. That is because, as I man sure you are aware from you experience with the MMCC, styrene and other plastics work best with flat plates, like a Mando, or used with Vacuum forming. The vast majority of Halo's armor, due to its complexity, is not conduciv around 2007e to vac forming.
My armor is classic Halo: Combat Evolved, which is very simple, and took to vac forming okay. Outside of me, I have only seen one group do vac formed plastic armor, for the Halo 2 Mark VI, but that was years ago, like around 2007. I have never seen someone attempt to do a modifed version of pep directly to plastic.
Short answer, you can use sintra or styrene or any other plastic, but you will not be able to use the traditional pep methods straight onto the plastic. At least not easily. But I would not discourage you from giving it a shot with something simpler armorwise, like a Marine, to try it out.