I know people freak out over flatworms, but my experience is that they boom and bust and will eventually go away on their own. A sixline will really limit what else you can keep in that tank.
Some of my favorite smaller fish are firefish, possum wrasses, ocellaris clowns, swissguard basslets, watchman gobies. There are a lot of interesting fish that don't get very big. I've been considering a passive-fish tank with nothing but small peaceful fish. I was looking at a group of masked gobies, too.