Unfortunately there are no triggers that could go in a tank that small. If you want to set up a small aggressive look into mantis shrimp. The smallest tank I would put a trigger in would be 75. Most species get anywhere from 8 inches to 24 inches. Some I am sure get bigger than that.
IMO There can serious problems when setting up a small aggressive tank. The inhabitants tend to fowl the water fast when they eat and poop. I really don't know much you could add to a tank that small.