I second what dudeguy said. I've heard that sixlines can be bullies in smaller tanks, even tho they're small fish. Mandarins need crazy ammounts of pods, which requires tons of rocks and sand, which a nano just can't support. I would agree that the shrimp would fight as well.
Maybe get 2 cleaner shrimp, a pair of clowns, and an anemone? BTW I would only do the anemone if you're advanced enough to keep one, and also are able to resist the temptation of adding tons of corals and other stuff. Now that I've got a 24g nanocube full of random soft corals and LPS and a few montiporas, I wish I went with a species only display, or a specific type of display, because now I've got the random coral collection in my big tank.