Let's see...

Outfit

1. A set of clothing, often with accessories.

2.a set of usually matching or harmonious garments and accessories worn together

If you wear any accessories with your kilt (kilt pin, belt, sporran, etc.) you are wearing an outfit. If you color coordinate your clothes to your kilt, you are wearing an outfit.

The kilt is not, by itself, an outfit. It is a garment. However, as soon as you don any other article of clothing, or accessory with that kilt, the kilt becomes a part of your outfit.

I never wear an outfit of any sort.
Guess what? A suit does qualify as an outfit.