Hi
As far as I know, you can show any dog you like at a companion show, and if it's purebred, you can go in the pedigree classes. As I'm sure you are aware, a longcoat is a fault - but I've been beaten by mismarked Ridgies at exemption shows and my aussie was given a best in show but I'm pretty sure the judge didnt' know what she was :)
Have fun
Carina