I think there is nothing wrong with wearing size variants of the same sett or items cut from different runs/ages of the same sett. I've seen pics of folks in the Highlands pairing different colour schemes of the same tartan (i.e. weathered with modern) or even different tartans from the same clan, though I personally don't prefer the way that looks.

Many people seem to likt a bias cut tartan waistcoat because of the contrast it creates with the kilt, but a square cut tartan waistcoat can also look good because of the way it fits with the kilt. I don't think there is a difference in formality of the tartan cut, but rather the height of gorge. A low cut waistcoat is for formal wear, while a higher cut one is a bit more versatile.

In my experience with THCD, a tartan waistcoat is uncommon for daywear, but not entirely out of bounds. I would tend to reserve it for impact, such as on Tartan Day or a special Scottish occasion. It would seem a bit much just for a trip down to the pub...

Here's a few pics of my tartan waistcoat, worn at increasing levels of formality.