Personally, I think you are not quite getting it. Lets see if I can put it another way for you. You have two fish currently in your tank whose minimum tank size is larger than the tank that you currently have. Therefore, you are overstocked! Stocking has to do with a lot of things besides just physical space; it also includes waste production, swimming space, etc. Just because the tank looks empty to you, doesn't mean you have more room for other fish. Mixing a bunch of aggressive, predator type fish and squeezing them in too small of space means you are going to start seeing more aggression, because they are unable to establish and defend territories, you are going to see fighting and ultimately you are going to start having fish dying.
The answer to your current question is simple - the simple answer is no additional fish is suitable to your aquarium given its current inhabitants.