I bought one of those tanks when they first came out, and while it worked fine and looked sleek and modern, the design of the tank makes it very difficult to scape, and very difficult to clean. I had to buy a very small siphon just to get around everything, and eventually I just went with an open top tank since I got tired of having to fidget around in between plants and driftwood.
I think tetras and other fish such as danios and white cloud minnows are a little too active for such a small tank, since they need more horizontal swimming space than the edge can offer. A small colony of shrimp would be great, or even something like the micro rasbora fish, which is more suited to nano sized tanks. I'm not sure about guppies, but I could imagine a small trio of females or all males (just watch for fighting) might be fine.
Shrimp are quite sensitive to water conditions, so in a tank this size, especially if you opt to do a fish-in cycle, you will need to keep a careful eye on your water parameters.