I have 8 serpaes, 6 black neons, and 7 blue tetras in a 20g, plus 5 corydoras and 2 otos. Overstocked by most standards, but this tank is also densly planted (75-80% of the substrate is covered) with 2 filters, and weekly water changes. So whether or not you could have higher numbers depends on how willing you are to put in the extra care. This tank is also over 3 months old and is very stable. I have even left it without attention for a week and a half (Vacation) and came back to a happy tank. I agree with Griz and would just stick with 3 species of tetra and perhaps either a small bottom-dweller or a peaceful centerpiece fish. Just slowly build up numbers.
What's the lighting in your tank? Vallis and Sword plants need adaquate light or they will not thrive. They will do even better, especially the sword plants with CO2 injection. I prefer the swords over vallis, but that is because once I inject CO2, my water becomes rather soft and vallis doesn't do as well. Tetras love a densly planted tank with swimming space in the middle.
A bit of information if you are interested. Tetras tend to school at the middle levels. Boehikea fredcochui (The blue tetra), however, tends to school in the upper levels of the tank and is rather like a danio in it's behavior. So if you're looking for a very active tetra that schools pretty tightly, I highly recommend the blue. It's subtle coloring will also look great in a planted tank, especially if you allow wood to seep tannins into the water, or if you add peat extract.
Good luck with your tank,
llj
