I have a one gallon tank on my desk at work. I have kept two plants alive for nearly two months with the 7 watt bulb that it comes with (not florescent) and the tank is fine. it has a UG filter and though right now I have a few baby platies in it, they are moving home once they do get bigger than a quarter inch, but they have plenty of room for now... In the future, I plan on keeping a Beta in it, with a few ghost shrimp. It will do fine in here too.
Since gouramis are very similar to betas, you could probably keep a true honey dwarf gourami in the 1 gallon as well. Kids in south america keep wild gouramis in jelly jars...