I believe a "ruche" is just a type of knot that is tied on an Ascot cravat and, as with any longer tie, looks better tucked into a higher cut waistcoat. Regardless of what you call it, the general opinion seems to be that it isn't up to snuff for traditional Highland formal. More like alternative/creative formal or perhaps a trend? Here is a slightly less scrunchy, black ruche that might be less objectionable though probably still won't pass muster around here:
With this I must disagree. Cravats can certainly be worn with Highland attire and while they have generally fallen out of favor to the likes of the necktie and the bowtie, they are still perfectly acceptable and are gaining more favor. Now I can agree that it they may not be considered "classic" but they do harken back to an older style, much as the jabot does. Besides, if I'm not mistaken, I've seen jabots worn with a cravat (but I may be wrong, and this wasn't with Highland attire.) Anyway, I think that cravats are well within the spirit of traditional highland attire, whether or not it has fallen out of favor and general use.