i agree it needs a school of something. i am a big fan of rainbows - a school of smaller rainbows like threadfin, blue-eyes, or boesmani would look great. the only potential problem could be the shark and gourami picking on the peaceful species. if you think that may happen, then go for a school of tiger barbs or, as you say, rummynose tetras would look great.
50gal planted - yoyo loaches, BNs, odessa barbs, giant danios
33gal planted - praecox rainbows, gold ram, bolivian rams, sparkling gouramis, licorice gouramis, lots of shrimp
2 x 5gal planted - bettas