This is by no means intended to sound mean or rude, I just want to pre-warn you. Twenty five pounds of live rock in a 55 gallon tank with aggressive fish is not going to filter the tank enough. When you think of "mean" aggressive fish, you need to think of huge bioload because that is what they are. Any type of puffer, lion, grouper, hamlet, eel, trigger, wrasse are all carnivores, which means that the waste that they put out is very large. You are definitely going to have to get about 40 more pounds of live rock in the tank if you are considering aggressive fish, unless you plan to do regular weekly water changes. Even with weekly water changes however, if you don't have the filtration, skimming, etc. to keep up with the fishs' daily waste, your tank levels will spike rather rapidly.
Before loading up with 2 to 4 "aggressive" fish (such as groupers or puffers), I would highly consider adding more live rock to the tank. This is definitely going to spare you the headache that may occur with only 25 lbs. of rock. Believe me, the extra rock definitely makes a difference. I'm not saying all this to be mean, I am just trying to save you the effort and money down the road.