Thanks for the opinions. With all that's been said, I think I'll have the guys in the traditional kilts with vests outside (that should also save them from having to get a belt). Probably black, with my best man in something different, with solid ties to match their kilts (all will be in different tartans).

As for the gents in greatkilts, though, because of all the gathered fabric I still think it may be better for the vest to be inside, either that or we'll have to make them vests that'll work with the greatkilt.