Don't think anything that has a pin on the back of it is gonna be too heavy for a kilt....okay a well made wool kilt.
Yeah, it would take a pretty darn heavy kilt pin to cause a well-made heavy wool kilt to start pulling out of shape. Especially if you have the double thickness at that edge of the apron for a double fringed edge. There's lots of strength along that edge.

As for it banging against the leg when walking, simply move it up. A kilt pin doesn't have to go at the bottom (and in fact, I kind of like it better when it's no lower than 1/3 of the way up from the bottom).