I would use the fiji pink ultrafine. I like smaller particle sands. In terms of allowing detritus to penetrate the sandbed, smaller grains generally restrict while larger grains tend to accept particle settling. In my beds with coarse aragonite, there was never any dust clouds on top. I could see the crud lining each chip from the sides of the glass, and it went all the way down to the bottom of the tank. Instant cross-section verification!With live sand, brown patches of light detritus were easy to filter out before they could penetrate the upper cm of the bed.
It all depends on the amount of maintenance and stocking/feeding you plan on working. IMO, the best way you could design that Nano is with a very shallow bed of that fiji pink. Disturb it greatly and siphon it regularly at water change, keeping all the dust/detritus out of the rockwork. This design allows the most flexibility with fish stocking, whereas a deeper sandbed nano needs to have more restricted stocking and much more careful feeding/export.
Welcome to our forum, glad to have you onboard.
Brandon M.