I don't think there's much of a problem with wearing a dirk with a Prince Charlie (as long as you remember it is worn on the right side of your body-- the sword goes on the left side). I think Kyle has made a good point about wearing a jabot and (if you want) lace at the cuff, especially if the groomsmen are going to be kilted and in black tie.

One of the things you will want to check is the rise of your kilt (an inch or so above you navel) and that your waistcoat comes well over the top of your kilt-- you don't want any "gaposis", especially if you are wearing a dirk as it will only accentuate the problem.

And as Kyle has pointed out, the dirk belt goes over your waistcoat, snug around your waist, not loose and resting on your hips.