Get a pair of Kribs
+1 for Herotilapia mulitspinosaFor a small but very sturdy fish that fits most any size tank, I have a favorite. The rainbow cichlid (Herotilapia mulitspinosa) is a very nice little fish that will harm nobody else in the tank. They come from small pons in nature and have adapted to fit the situation. As they often get very crowded, they have developed actions to prevent any real damage. They often do "bluff" charges but rather than running the lesser will often go into a head down submissive position and the dispute is settled. I have kept them with guppies.
It's just my personal preference, but I don't like angels in less than a 55. I love big angels, and big and 30 gallons just don't work. Smaller angels maybe, but smaller fish do grow bigger.Angelfish! Get 1 or a pair. There are one of my favorite fish and they actually have different personalities. Very beautiful and graceful. If you only have one they will not be aggressive at all.