I overstock and it has been very successful. As far as how many in a 56 I think others can better answer. I have close to 40 in my 90 gallon. It is still important to have a good stock with compatible fish and filtration is very important. Overstock = over filtration.
And you need a good ratio of males and females, 1 male - 4 or 5 females. No matter how much you overstock if you only have one male and one female of a certain species, that female will be constantly chased and pursued.
The theory behind overstocking is to spread out aggression so one fish can't be targeted. There will always be aggression, these are aggressive fish. But if there are 8 red zebras for instance, chances are the same red zebra will not constantly take the brunt of aggression.
Another important aspect when it comes to aggression is territory, have PLENTY of hiding places, caves etc.
Hope this helps.