Sorry, I just couldn't help it.

The wearing of a plaid is generally reserved for very formal attire, or when one is getting married. Other than that (unless you are in a band that wears a plaid as part of their uniform) plaids just aren't worn nowdays, except as noted above.

The plaid would be worn with one of the formal evening jackets-- dress Argyll, Prince Charlie, regulation (or standard) Doublet; Montrose, Kenmore, or Sheriffmuir doublet.

And no, I wouldn't mix tartans-- the plaid must match the kilt, UNLESS you are in the second touring company of the revival of "Brigadoon".