You're spot on....there are no rules as to what a kilt pin must look like. I find that pins that are personally meaningful are my favorites. I hardly ever wear my old traditional sword pin (But which I cherish because it was a gift to me from a member here years ago when I first joined the forum!)

In my experience:
A) Don't make it too heavy. If you are depending on the weight of the pin to keep your apron from flappin in the breeze, you have too little kilt or too much wind.
B) Watch out for protruding points and edges that may catch on things and rip your kilt.
C) Consider using the cool new super-magnets to hold it on. Two magnets glued to the rear of the pin and a rectangle of steel behind the apron will hold nicely.

A set of pins for your groomsmen could be ho-hum or something really special and meaningful. Good on you for wanting to step outside the box.