Thanks for all of your responses and concern. The QT tank sounds like a good idea Hatchet. I guess you never know what the fish are bringing to the other fish when you add one.
Since others have had good luck with gouramis and high pH I think I will try again - the guppies, platys and barbs all seem to coexist well with a gourami.
Capekate - the plant looks great and is a good addition to the amazon swords and java fern that I have. The elodea doesn't last long with my pleco

so a floating plant might work better.