Six line could work, but keep in mind that they are aggressive, so if you're set on having one in this tank, make it the last addition.
Fish that would work (not all together):
Green banded goby
Tail spot blenny
Bi color blenny
Pair of Ocellaris clownfish
Firefish
Royal gramma (or other basslet)
Any of the smaller gobies or blennies
These are the stocklists I've had in my 14G at various times to give you some ideas:
1:
2 ocellaris clownfish
1 green clown goby
1 skunk cleaner
Various snails and hermits
2:
1 ocellaris clown
1 Yasha goby w/ pistol shrimp
1 tail spot blenny
1 fire shrimp
1 pom pom crab
Various snails
3:
1 Yasha goby w/ pistol shrimp
1 orange spotted filefish female (ate frozen)
1 tail spot blenny
1 fire shrimp
1 pom pom crab
Various snails
4:
Pair of orange spotted filefish (ate frozen)
Blue spot jawfish
1 skunk cleaner shrimp
Various snails
5:
1 six line wrasse
1 fridmani dottyback
1 baby starry blenny (would be moved to a larger tank as it grew)