I wouldn't get an active swimmer over 4in, or a bottom dweller/elongated fish over 5-6in. I'd go for some of these fish.
•Neon Goby (Elacatinus Oceanops) or other gobies of the genus Elacatinus
•Randall's Prawn Goby (Ambyleotris Randalli) or other gobies of the genus Ambyleotris
•Hi-Fin Redbanded Goby (Stonogobiops Nematodes) or other gobies of the genus Stonogobiops
•Yellow Clown Goby (Gobiodon Okinawae) or other gobies of the genus Gobiodon
•Court-Jester or Hector's Goby (Koumansetta Rainfordi or Hectori)
•Yellow Watchman Goby (Cryptocentrus Cinctus) or other gobies of the genus Cryptocentrus
•Masked Goby (Coryphopterus Personatus)
•Bicolor Blenny (Ecsenius Bicolor) or other blennies of the genus Ecsenius
•Canary Blenny (Meiacanthus Oualanensis) or other blennies of the genus Meiacanthus
•Chalk Basslet (Serrans Tortugarum)
•Yellow Assessor (Assessor Flavissimus) or other assessors of the genus Assessor
•McCosker's Flasher Wrasse (Cirrhilabrus Mccoskeri) or other wrasses of the genus Cirrhilabrus
•Blue Flasher Wrasse (Paracheilinus Cyaneus) or other wrasses of the genus Paracheilinus
•Helfrichi Firefish (Nemateleotris Helfrichi) or other firefish of the genus Nemateleotris
•Lyretail Dartfish (Ptereleotris Hanae) or other dartfish of the genus Ptereleotris
•Yellowheaded Jawfish (Opistognathus Aurifrons) or other jawfish of the genus Opistognathus
•Fireball Angelfish, Flameback Angelfish, or Cherub Angelfish (Centropyge Aurantonotus, Centropyge Acanthops, or Centropyge Argi)
•Banggai Cardinalfish (Pterapogon Kauderni)
•Yellowstriped Cardinalfish (Ostorhinchus Cyanosoma) or other cardinalfish of the genus Ostorhinchus
•Red Spot Glass Cardinalfish (Apogon Parvulus)
•Blue Eyed Cardinalfish (Zoramia Leptacantha)