Any updates on your tank?
For pico tanks, we have all been advised to keep the number of fishes to a minimum because we are limited by the small volume of water. Naturally for a tank of the above size, we probably shouldn't keep more than one or two small fishes, although it also depends on the species eg. no tangs. I have a tank of similar dimensions as yours and I have 1 clown, 1 Hector's goby, 1 blue neon goby, 1 red spotted goby and 1 six line wrasse. The key is to choose the smaller fishes that can be found. I don't know if I will be adding more fishes but controlled feeding and frequent water changes help to maintain the quality.