This is a touchy subject that is debated all the time. IMHO and IME I don't believe you can over skim a tank. You could put a 5,000 gallon rated skimmer on a 55 gallon tank. The end result would be a 5,055 gallon system. The skimmer would only skim the amount of proteins and waste that the 55 gallon tank made. And if you under size a skimmer, it will work all the time never really getting everything out of the system leaving you with issue's.
And indeed I agree. All skimmer's are under rated by at least 50%. So if it say's it does 100 gallons, it's only going to keep up with a 50 gallon tank with a very small bio-load. Like 1 fish.