It is never a bad idea to contact GMLs before you start just to get to know folks in your area. Some garrisons do armor parties, and the ability to get in-person advice on the actual build process is always a benefit. Plus, you can at least start to get to know the folks in your area and make it a smooth transition into membership after approval.
It is always a VERY good (dare I say, should be required) step to contact the Detachment that you are building the character for, since we are the specialists for details and the nitty gritty that the general Legion wouldn't have.
With regards to the approval of sintra and styrene, my only comments are that it comes down to being able to replicate the armor from references. The important thing should be the quality and surface finish of the final product (lumps, dents, divets, bubbles, etc that would alter the surface of your armor), troop-ability, durability, and weight. As a MM, you probably have already seen the differences that people have provided between ABS, resin, and metal armor in those areas. If it is meant to be armor, it shouldn't bend when you walk (hence, no foam). It should have a smooth surface overall, with some weathering. It should take into consideration where you live (certain plastics react and age differently in colder climates).
So, the short answer is yes, those materials are approvable so long as you are able to replicate the details and structure of the references while also taking into consideration wear and tear in your climate.