"If I go and resize, do I have to resize everything I find in the texture folder for that item by the same percent, or just certain files?"

You can if you want and keep a separate folder for the reduced textures. I reduce all my sets and save them on a disk to use when needed. For animations, I'll reduce the smaller files to 512 and sometimes even the main textures to 512.

This should help the performance of the firefly render so youi can still have those P6 advantages.