I think Boraras Briggittae or however it's spelled is going to be too small. You'll loose the effect of having them in a tank of that size, even if you added around 30, which you could. But by adding that many, it would look to kitschy, in my opinion. I would go for a good shoal of maybe 15 or so ember tetras. Tiny enough, yet large enough to have a good impact on the eyes.