Generally at a wedding only the groom wears a plaid. That being the case, you would probably be best advised to forgo the fly plaid.

Medals: the conventions are the same for civilian attire, Highland or otherwise. If the wedding is taking place before 6PM then full-size medals are worn; after 6PM miniatures are worn. If wearing full-size medals at the wedding it is not necessary to change to miniatures later in the evening. With Highland attire the placement of medals follows the British custom with the medals pinned to the lapel approximately 1 1/2 inches below the notch. Chas' medals are spot on, and exactly how yours should be worn with civilian Highland attire.