This photograph shows a red velvet Sheriffmuir doublet worn with a three-button waistcoat, made in a matching tartan to the kilt. I don't think it looks odd, at all. It's being worn with a lace jabot.


Of course, the Sheriffmuir is more commonly worn with a higher waistcoat. I don't think that makes it any less formal in this instance.

Here is an image from Highland Clans that shows it with a five button tartan waistcoat and bow tie.


This image shows it worn with a five button tartan waistcoat and neck tie (note the wing collar shirt).