1. There isn't a hard, firm, it must be this as it is interpreting a 3d model and texture maps. I'd recommend something that fits well and has some stretch to match the look of what is seen in the animation for the grey and black parts. Be sure that the colour varies, to look like how it is presented, as best you can.For the red, my wife used a crushed taffeta. If you'd like there are some who jumped in, but her build thread is here:
2. For the medallions and greeves I used Sintra (Extruded PVC) Other things could work, but it worked well for us. For the straps we used leather that we covered in the cloth materials to match the textures.
3. Sure. So long as they lay flat and match your makeup well enough to not distract that should be fine.