Yes! A brackish water aquarium is a great way to use a 10-gallon tank.
Swamp guppies (Micropoecilia picta), Endler guppies, wrestling halfbeaks, BBGs, rhinohorn gobies, desert gobies and most of the flatfish would all be obvious choices for such a tank. Add some nerite snails and algae shrimps, and you'd have quite a fun little community.
Do see the "Small Fish" section of my FAQ for some more ideas.
Cheers, Neale