The word plaid or plaide is just the Gaelic word for blanket. That etymology alone should allow one to understand the nature and origin of the garment. The Fèileadh Mhòr (literally "big or great wrap") was typically two 4-yard lengths of 27" fabric sewn selvedge-to-selvedge to make one 54" wide by 4-yard long piece of fabric. This was pleated (more like gathered, really) by hand and then worn belted.
The term Great Kilt is just a translation of the Gaelic as the word kilt itself is believed to come from the Middle English kilten = to tuck up.
The Fèileadh Beag ("little wrap" - anglicised as philabeg) was just the bottom half of the Fèileadh Mhòr with the top half worn on the shoulder still as a shawl or blanket. The closest descendant of that item today would probably be the drummer's or piper's plaid. The fly plaid, while still usable as a shawl if left unsewn, is a bit too small.
The Ancient Kilt you're referring to is probably the bizarre tartan "toga" styled garment they devised for the Braveheart film. I'm not really sure why they did this and didn't just use the Fèileadh Mhòr (even if it would have still been historically inaccurate).
As for Highlanders not bathing frequently that is simply not true. Highlanders, and by further extension Celts, traditionally prided themselves on cleanliness (unlike most Continental Europeans at the time - no offence!) If fact, they often removed much of their body hair to aid in this process - leaving only a handle-bar styled moustache.
I suspect this kind of thinking is a left-over bit of propanganda from the usual source. It makes one think of stuffy professors with lit pipes discussing "hairy Celts!" Harumph! Harumph! Similar thinking led to the 19th-century illustrations depicting the Irish peasantry as chimpanzee-like figures.

Sorry I got a bit side-tracked, I hope its illuminating!

Is mise le meas,

Seán Liosliath Ó hAirt