With a good filter and slow stocking you could get away with a betta and maybe two panda cories, or two otos, or a largish snail. The tank has a small surface area, so you don't want to overdo it. I'd let the betta be your showpiece, and use decoration (rocks, driftwood, real/plastic/silk plants) rather than more fish to make the tank pleasing to look at. I've seen gorgeously scaped 5g hexes, but it takes imagination.