If I am a tag size 32 in the 21 oz 666 cut, would I be the same for the 25 oz or are people sizing up due to shrinkage?

Just a couple questions on sizing for this item. I'm a 37 inch chest, 5ft 10 inches in height. I'm a medium comfortable in all Iron Heart Westerns & size small in American brands. If I want to be able to layer a thick sweatshirt or UHF under this jacket, would I be best with a small or medium? I see AJ wears a medium for this jacket but then in the olive color took a small so I'm a bit confused on sizing as I'm closest to his specs.

If I am a size 10 Brannock sizing, would I be a 10 in these as well?