If you have a brooch with a very large pin, I have been told that it was originally designed for a pipers plaid. Brooches designed for the PC, will cost more and have small pins.

I have two brooches that have very small push type pins, and I either pin them through the seam in the collar of the PC or sometimes on the epaulette.

If your brooch has a very large pin, you can do as stated above or, if the brooch is worth it, you could take it to a jeweler and have small pins attached, or if you are adventurous, you could cut off the big pin and epoxy small needle sized push pins.