I think that you can keep a pygmy pufferfish in that size. And they are CUTE! They are also pretty smart and have very curious personalities.
I'm not sure if you can keep anything with them though. Definitely nothing that swims in the mid or top levels. If it's heavily planted, you may have luck with a couple of bottom feeders. Google the fish, I think some people keep an otocinclus or two with theirs.
This is a good resource for it (since I can't add the link... I'm going to spread out the letters, lol) - d w a r f p u f f e r s.c o m