Hi, according to the calculator at the top of the forum and the instuction manuel for your tank (found a pdf online) your tank is 39L or 10G (using their dimensions of 32x32x38cm) now I'd reckon a little small than this to allow for displacement etc. Maybe 35L, 9G.
The rule of thumb is 1 inch of fish body per gallon of water.
4 glowlight tetras = 4 inches.
2 bronze corydoras (which grow to 3 inches each) = 5 inches.
So with what you have at the moment you're done. 4 + 5 = 9 inchs of fish, for 9G of water.
If you took those corydoras back and swapped them for pygmy corydoras instead then you could have more. Pygmys only get to an inch long each. You could have 3 of those, along with another couple of glowlights, and then you'd be at capacity.
It's not a case of whether the upper/lower levels get crowded (although that does have something to do with it in larger tanks), in your case it's what is too much for the filter to handle. Under gravel filters are not the best anyway, and I personally wouldn't want to push it by overstocking.