Guidelines rather than rules I suspect comes closer to the truth of it for us non-military types. As an artist, I suspect these evolved guidelines boil down to a matter of proportion. Why a wide belt and not a skinny one? Why a hand width gap between the bottom of the belt and the top of the sporran? Why a short waisted jacket and not a longer one? Why hose at four fingers below the knee and a kilt at the top of the knee? I suspect that much like the golden rectangle, these proportions have been judged more visually pleasing than all the attempted alternatives.

Bend the "rules" break the "rules" if you'd like I suppose. Why not - after all that's what fashion is all about.

Now for the matter of rubber chicken sporrans? I believe they are Dadaism pure and simple.