A good thing I all that know is that you could easily up their numbers to 8 or 10.. and hopefully you got a plant that doesn't need soil IE Java fern, mosses or hornwort. I bought a java fern 2 weeks ago and it hasn't died yet!
If you add several java ferns and hornwort they could easily absorb the extra wastes.
Black skirt tetras are really really hardy WAY hardier than any other tetra I have come across. Neon tetras are not hardy at all. You can't go wrong with them as long as you change the water without scrubbing and bleach... or keep them with predators. They eat like piggies and grow to 2.5 inches... and are really deep bodied.
Also they are pretty little fellows.
I want to see your stocking list to make a good estimate of your limits. IN MY ARROGENT OPINION the less species in a tank the better if they are gregarious. IE all characins, 99% of cyprinids, some syno, all cories, monos...
I say max 3 species of shoaling fish in a tank of 29 gallons and maybe 4 for 55... ANYWAYS the black skirt tetras will live alright in a group of 4 but they will be prettier with more. I dunno if they are as dependent on others as much as other tetras, I had 3 for a few years when I was a total nub. Why yes, I did scrub out the filter every water change! (12x per year)