Excuse my intrusion.....
But my first saltwater tank had only a yellow watchman goby and his shrimp buddy in it.... And two snails. I started out with freshwater too (and I still prefer it to SW
, lol, just easier) and I know you're thinking that one fish is NOTHING, but believe me, they are SO interesting! They live in a hole right with the shrimp, and the two become best buddies.
It's awesomely fun to watch. There are many goby species that will pair up with a shrimp, but I'm not really sure which are more suitable for a smaller tank.
Good luck with your tank!