It sounds extremely lightly stocked. :p In a 55 gal you could double that without wincing.
Again, check your water hardness - it can be just as determinative as pH, though since you have a slightly acidic pH it is probably softwater. :) Your species choice is therefore pretty good, working on that assumption. :) For more bottom-dwelling life, you could also look at kuhli or pakistani loaches, or have two schools of different cory species (though one large school would make the fish super happy).
For a bit more colour, consider looking at other kinds of tetra - glowlight, ember, cardinal and green should all work out. :) Personally, I think that people who turn their noses up at tetras are missing out on the beauty of a school of 20 cardinals.
I would also suggest using black substrate (sand is best for the cories), as black makes both plants and fish really pop. You'd have a truly beautiful display that way. :)