As far as the uniforms go, mainly because even though they were "issued them" if they got destroyed or were deemed unfit for wear while on duty, then any replacement would be taken out of their pay, just like the regular regiment. HOWEVER......

It is not being said they kilts weren't worn by the Darien scots when they arrived, only that the only DOCUMENTED proof was that of the Highland Independent Company. Seeing as haow the majority of the settlers came from the Inverness area, one could assume, and I would agree to a point, that the majority of them had some kind of simple kilt to their name, as at this time Highland wear had not been outlawed (1733), so looking at what people wore in Inverness at that time could be carried over to the Darien Settlement. However, you would have to take into consideration the long travel time at sea, what that would do to a kilt, how uncomfortable it would become, and how easy it would have been to just start wearing regular britches and such, and keeping the kilts for special occasions, as seems to have happened later during the French and Indian war.

Cheers,
Chad