My wife is the same. The real problem is that if I focused on matching the kilt, I could only ever wear green, blue, red and tartan/check pattern.
I’ve been trying to harmonize everything else while treating the kilt almost as a neutral (without ignoring it completely). I find that the results have worked well for the most part.
In regards to the vest and size of the diamonds. I like that the difference in size creates a contrast between my upper and lower halves. Kind of how a belt or waistcoat helps divide a kilt-wearers body. If the diamonds were smaller, I wonder if it would look too much like the tartan and create a weird “tunic” effect?
Bookmarks