I have a 15 gallon AIO that's home to a Court Jester Goby (
Koumansetta rainfordi), which is a tank raised fish from Biota, and I'm really happy with him. Eats everything, super easy to care for, unlike the wild caught ones.
I also have a Yellow Watchman Goby (
Cryptocentrus cinctus) that's paired with a Tiger Pistol Shrimp (
Alpheus bellulus). These guys have a cool relationship, the shrimp works all day to maintain a borrow it shares with the Goby, and the Goby - true to its name - spends all day watching for danger. If the fish gets startled, it slaps its tail, which alerts the shrimp, and they both go darting in their borrow.
Sadly, I also had a Tail Spot Blenny (
Ecsenius stigmatura) until just yesterday :-( I accidently left the screen top off of the tank late the other evening, and I found my poor Tail Spot Blenny, Lenny, on the floor yesterday morning. RIP, Lenny! Sorry, little dude!
I went to the LFS tonight and replaced the Tail Spot Blenny. He's in a QT tank right now. I would suggest that when you bring new fish home that you quarantine them to reduce the chance of fish parasites getting into your main system.
I put a fair amount of time and research, and asked a lot of questions here on R2R when I chose these three fish for this small tank. I wanted ultra-peaceful fish, including around inverts. These three have done very well for me in my 15 gallon, with the exception of the poor Tail Spot jumping out of the tank.
Additionally, I have a Fire Cleaner Shrimp (
Lysmata debelius), a couple small Emerald Green/Mithrax crabs (
Mithrax sculptus), a variety of hermits, snails, etc. I started the tank with maricultured live rock and sand from Tampa Bay Saltwater, which came with A LOT of cool hitchhiker critters.
So no little shoaling fish like freshwater Neon Tetras (
Paracheirodon innesi), but a pretty interesting cast of characters for sure
Good luck with your tank!