I think that in determining fish load you need to take all those things into account. Some fish require the whole tank to themselves because of their "personality" others are community fish and get along with everyone. Others start out great and get more aggressive as they get bigger. I think its a matter of knowing your tank and seeing how they all interact before you add another fish. For me I like to think I take those things into account but occasionally I make dumb moves that turn out okay...mostly.
Its unlikely that I'll be adding another fish anytime soon to my tank (although don't hold me to it
) but I take into account how much the fish eats (and how much comes out the other end), max size, whether its a swimmer or a sitter/hoverer, color (I know this sounds stilly but no more blue fish, my regal gets quite ticked) also what the increased bioload may do to the tank. I do 10% water changes every week to suck out all the detritus and get some fresh water in there which I think has helped a lot in making my tank successful....mostly. Sure I've got to put up with wafer algae and caulerpa gluing my rocks together but I just don't look at it as much
