Hi all,
A pair of Rams are a possibility, you may need to pair them first in a larger tank. Apistogramma borellii would also be a possibility (A borellii needs a cave to spawn, I like the 1/2 coconut planted with moss). I like the very large amount of "java" moss approach for small spawning tanks, and the development of "biofilm". This is the even smaller tank (in the lab) that I used to keep a pair of A. borellii (and fry) in (the light is the Boyu 3 x 8W).
Other possibilities would be any of the 3 dwarf Corydoras species (C. hastatus, habrosus or pygmaeus), they are small enough that you can keep a group of 6, and reasonably easy to spawn. If you keep them with a group of Otocinclus, these are more likely to spawn as well.
The Dwarf Sparkling Gourami (Trichopsis pumila) is another good one, or the Clown Killii (Epiplatys annulatus), all these possibilities so far can raise some fry to a reasonable size in the tank with the parents present.
For Galaxy Microrasbora or any of the smaller Pencil fish/Tetras/Marbled Hatchet you need to exclude the parents from the tank after spawning or they will eat the eggs/fry.
I'm going to try Threadfin Rainbows (Iratherina wertheri) in my shrimp tank, and I'm hoping that these will form a self sustaining colony (the fry are apparently very small but I'm hoping they will be able to find enough rotifers etc to pick at from the biofilm on the moss).
cheers Darrel