Ahh yes, I remember now. lol Soo many threads I try to keep up with sometimes I forget.
I think the fuge won't generate enough pulling power to pick up much sand, so I think you should go with whatever sand or substrate YOU like. I would stay clear of crushed coral is all. Too large particles in my opinion and too likely to harbor nitrates and other nasties.
Not sure on the octopus, I would think it would pull a decent amount of water but only when it return into the tank could there be sand issues. A well placed rock can help on that.
I have an ac70 running in my 20L and the input tube is almost to the bottom of the sandbed, and I used very fine black sand for that one. I placed some rocks where the water flow comes back in and ta da no more sandstorm or sand moving and making mounds.