Overstocking a tank means your gonna be fighting a constant battle with water quatlity as jason said. More water changes will be needed. Some fish need space and putting them in a small tank will stress them to death.
Saltwater fish by their very nature are more agressive than most freshwater fish. This means they need more room. Saltwater also has less oxygen than freshwater.
Experenced hobbists can fudge on the rule of thumb of 1" per 5 gal
but beginners should have a lightly stocked tank till they get hang of feeding and water changes and water quality.
Saltwater fish are used to living in an ocean and thats is how they have evolved. Water conditions in the ocean changes very little over the corse of the fish's lifespan so they are much more sensitive to water qualtiy changes.