Isn't the "flat in front, pleats in back" just a byproduct of putting on the breacan-an-feileadh?

At least one that doesn't have a drawstring and is just a rectangle of cloth.

When the "little kilt" was introduced there wouldn't be any reason not to pleat it all around.

I wonder if there was a military influence which helped the "flat in front" small kilt become the 19th century standard.

I say that because when the military dropped the belted plaid and began wearing the small kilt in Full Dress they did so in conjunction with a new garment devised to imitate the look of the belted plaid. It amounted to the upper half of the old belted plaid, ending at the waist, and with a narrow self-belt fastening around the waist (the belt being hidden under the jacket).

I've read that in 1794 this switch took place for enlisted men but for a time Officers continued to wear the old belted plaid. Thus with their new imitation upper-halves, and small kilts with flat fronts, the enlisted men would resemble their Officers wearing the full belted plaids.

(I can't find a date for when Officers' belted plaids were likewise abolished for Full Dress, but I think it would have to have been around 1800. The Officers too wore the new upper-half belted plaids as soon as they switched to small kilts for Full Dress.)