Well, I do not know about whether you have thought of it but do you think you would like a small school of tetras? It would probably take up all your space. There are even some types of tetras who are not the schooling type according to the book I was looking into (Eyewitness Handbooks
Aquarium Fish by Dick Mills)
Pretty Tetra
Rummy-Nose Tetra
Bleeding Heart Tetra
Serpae Tetra
Emperor Tetra
Cardinal Tetra
Or have you thought of some Cories? Or the better choice (because they are not schooling) are Otocinclus affinis.
(Otos, of course

) They are the smallest of the catfish that I could find for bottom level dwellers.
The platys occupy all levels and the gourami will occupy mostly the middle and upper level due to having to breathe at the surface. (Labyrinth fish)
If you check out the tetras, they also occupy all levels of the tank. Although the ones I have seen have mostly been at the middle of the tank.
Someone else may have differing information so this is only one suggestion.
Rose