I have a Court Jester Goby (AKA Rainsford's goby,
Koumansetta rainfordi) and I love my little dude! I got mine from Biota, which specializes in tank-bred and tank-raised fish. When I was considering my purchase, many people reported that wild-caught Court Jester Gobies were difficult to feed, but the tank-raised Court Jester Gobies from Biota seem to be a different story. My tank-raised Court Jester eats everything including frozen, pellets, and even flaked foods, plus I see him nipping at the rocks. I'm very happy with my Court Jester from Biota. if you're still considering one:
https://shop.thebiotagroup.com/products/court-jester
Or, if you're looking to do a small goby and pistol shrimp combo, I quite like the Yahsa Goby (
Stonogobiops yasha) and Randall's or Candy Cane Pistol Shrimp (
Alpheus randalli).
Another peaceful nano fish to consider is the Tail Spot Blenny (
Ecsenius stigmatura).
I have a 15 gallon AIO with a Court Jester Goby, a Yasha Goby, and a Tail Spot Blenny. This has been a very peaceful community for me. The fish leave each other alone, and they don't bother inverts either.
Good luck with your Fluval 13!