What would lead you to think you need that much filtration on a 180g. IMO scrubbies are not ideal for submerged media and work much better in a wet/dry setup. A better choice would be Bio max or Ehfisubstrat. I don't own a FX5 so I'll let others comment.
Bro i aint no pro but i have 1 FX5 on my 125 and my Sh*t is crystal clear, and i have a bunch of fish in the tank, all i have in the FX5 is the white filter pads the fx comes with also the blue fine filter pad, pre-filter, and the amo/carb chip from api. thats just what i do tho..
I plan to just use scrubbies and bioballs for the center of fx5's and the regular sponges so i dont see why not... just clean the sponges and let the scrubbies n bioballs accumalate beneficial bacteria