I tend to think of all brooches as pins, but not all pins are brooches, depending on their size, shape and function. In highland attire, a brooch might be used to secure a plaid, but would probably rarely be worn below the shoulder. The kilt pin is usually attached on the lower corner of the outside apron, ostensibly for weighting it down, but really just for show.

Any bit of costume jewelry that is attached to clothing with a pin and clasp can be considered a brooch. Fellers tend to eschew anything that is too terribly ornate. Slim, elongated, and lightweight - might make a good kilt pin. Heavy, chunky, and compact or round - probably a brooch. A brooch worn on the lapel or shoulder is a brooch. Move it down onto the kilt, and you've got a kilt pin, unless it is huge and meant for a plaid - and then you've got a injury waiting to happen.

Regards,
Rex.