If you go with a trio of male guppies then pygmy cories would be a nice bottom dweller. Kuhlies would also work, as would otos.
If you didn't go with guppies you could get a betta as suggested, and you should be alright with any of the 3 bottom dwellers I mentioned.
Some tetras would work. Neons in a group of 8-12 with nothing else in the tank would be a lovely sight with the dark substrate and even mroe so if you went for a black background. you could also try glowlight, harlequin rasboras or if you keep the temperatures low enough you could try white clouds.
Tetras can be sensitive depending on species, but if you are waiting until the tank has cycled (my preferred choice) then it shouldn't be a problem. You could potentially use the Goldfish
to cycle it but in a small tank, with Goldfish
being such large waste producers, I think it would be a bit unfiar on the fish as the levels would rocket. Better fish for cycling are danios, thoguh as i mentiuoned the fishless cycle is my preferred choice. it's much kinder, and easier ImhO because you don't have to keep such a close eye on the params as there are no fish in there to harm if they do go through the roof for a bit.