William,

This is from Lord Lyon's website:

Armigers
A person who has registered his or her own coat of Arms and Crest, or inherited these according to the Laws of Arms in Scotland from an ancestor who had recorded them in the Lyon Register, may wear their own Crest as a badge:

  • either on its Wreath, Crest Coronet or Chapeau,
  • or, as is more usual, within a plain circlet inscribed with his Motto.

An armiger may also choose to wear instead the Crest badge of his Chief if the armiger is a clansman. An armiger is entitled to one silver eagle's feather behind the plain circlet, and if he is also a Peer he may add his appropriate coronet of rank on top of the circlet.
In essence, if an armiger does not have Scottish arms recorded in the Lyon Register, then he should not wear a feather. My arms are not Scottish, so I don't wear a feather.

Regards

Chas