I had a pair of oscillaris clowns in my 8 gallon biocube at work for about 2 years before I gave them away with the frogspawn. Conventional maybe, but there's a good reason they're popular. They don't need to be paired up, but their little dating rituals are fun to watch.
Cardinalfish like hi fins (banggais) and pajamas would work well in a nano - they like low flow, and aren't too aggressive. Just make sure they're eating at the store and buy captive bred if possible. Same goes for clown gobies - I had a wild-caught one that got little white bumps, which I think was a symptom that it was malnourished. I figured out too late that it was just "pretending" to eat...it would dart around grabbing food like crazy but it was spitting it all out.
Not sure mandarins are a good beginner fish, only because they're constant feeders and will likely starve to death without multiple daily feedings of live brine shrimp (at least until they learn to eat prepared foods.) If you get an ORA "aquacultured" fish it will probably eat prepared foods already. They also jump, so you need a good cover. Challenges aside, they are colorful, active, and even breed regularly in small tanks if you get a pair, so the rewards are probably worth the effort to some.