20 gal is pretty small... You could have ONE Figure 8 Puffer (they will kill anything else) - OR - Knight/Fan Dancer Gobies (tank good for 5 maximum) - OR - Bumblebee Gobies (tank good for 15 or so maximum).
Knight Gobies and Bumblebee Gobies can be mixed maybe, but obviously this means you can't keep 5 or 15 of each, adjust numbers accordingly.
There's not really many other Brackish fish I can think of that are good in a 20 gallon.
I keep all the fish mentioned above (in various tanks), also keep Green Spotted Puffer, Violet Goby, Green Scat, Silver Scat, Archer Fish and freshwater
(don't let the name fool you, they're Brackish) Flounders. In my opinion none of these fish should be in a 20 gal.
I hope that helps narrow things down.
You can also research for yourself on Glass Fish for Brackish... Some of those might work... And if you hold your salinity over 1.010 consistently you can also add Marine Blue and Red Legged Hermit Crabs. I have successfully kept them in 1.012...