These rules are supposed to give you a very rough estimation of what to do. Sometimes they are just impractical or mean nothing.
Aggressive fish require much more space than mild fish.
Common sense should also be used. Don't touch the fish with dry hands. You will severely damage their slimy coat which is both a swimming aid and a protection against diseases. Once you net the fish always wet your hands before touching it. When you have to touch a fish with open wounds it would be preferable to use surgical gloves which have been previously washed to remove the talc dust.