As for your questions, I would do 1 basket of quilt batting/scouring pads (quilt batting is cheaper and IMO better, much better) and 2 baskets of eheim ehfisubstrat (or however you spell it! silly germans) in each FX5... They already have sponges surrounding the media baskets for mech so a whole FX5 of mech would be a waste IMO.
Talk to JGray12 about an fx5 mod that he did... something like doubled the media capacity of his fx5's.