My tank is two feet tall, 1 foot wide, and each side of the hexagon is eight inches roughly
You want to look at less active swimming fish, and there are a number of species from the Amazon basin that fit this requirement. I will just list species that occur to me, you can look them up on Seriously Fish to see photos and find data. But all of these will do quite well in a "tall" tank. If you can find branches to place from bottom up the tank near the top you will have a perfect geographical aquascape fort these fish. Lower plants are not needed, but floating are as these fish live in flooded forest and small blackwater or clearwater streams that are dimly lit.
Nannostomus eques [this fish remains at the surface and swims in an oblique angle so ideal for upper level]
Rineloricaria parva [the common and smaller whiptail]
Obviously not all of these, but any of the above can be combined with no issues. All are shoaling, except the Rineloricaria, and somewhat the Chilodus and Characidium, so a group of the species is necessary. Numbers will depend on the final species selection, but it is always better to have more of each species with fewer species than the reverse.