[Buccaneer]

It depends ... I think you really have too many factors to make a broad statement as to this many inches of fish per gallon

Bottom line is filtration

if you can remove as much fish waste as the fish produce then your tank can support whatever that number turns out to be ... only have to look at commercial fish hatcheries to see mega-filtration in action. A larger more efficient skimmer will allow you to keep more fish than a smaller skimmer or no skimmer as well as more live rock.

the other thing to consider is if you are stressing out certain kinds of fish either in closed quarters or forcing them to share space with incompatable fish

You have to look at your own tank and pay close attention to what happens when you add another fish ... I have tried to resist adding more than one fish per month to allow the biological/mechanical filters to catch up to the increased bioload.

Cheers