Interesting thread.
It does sound like the aquarium in question is overstocked, I hate to say. This is extremely easy to do, and I've been guilty of it myself. I used to overstock my tanks to nearly 200%, thinking that live plants added lots of leeway to general stocking guidelines. It turns out that this isn't true, because TDS (total dissolved solids) and DOC (dissolved organic compounds) still remain, even though ammonia, nitrite, and nitrate get absorbed by the plants. In an unplanted tank, nitrate levels are indicators of DOC, but the correlation is thrown off with live plants, making it very hard to measure some of the toxic stuff in the tank.
After learning about this, I reduced the number of fish I keep in my tanks. I have to say, an understocked tank is less stressful to maintain, because I know it's going to be able to hold its own for a long time. The fish all seem happier, too, and are consequently more fun to watch.
I've had terrible luck with Pearl Gouramis. Three died within 48 hours of purchase, one died after about two weeks, and my only one that survived for any length of time was outrageously aggressive. I've heard that gouramis are peaceful and hardy; I hope my experiences were out of the ordinary...
Neon Tetras are apparently finnicky fish not well suited for beginners. I had over twenty of them, and finally returned the last seven or so survivors to the LFS.
With my tetras and gouramis, all my water parameters were well within acceptable ranges. Hopefully you'll manage to figure out the problem in this case.
Good luck.