Guild Wars 2

Basically, if I buy a monocle of say, medium armour on the tp and salvage it for the skin, will I unlock the skin across all armour weights or just medium?

If you salvage or bind it in any way (i.e. equip it), you will unlock the skin to your account bound wardrobe, which can be accessed by any of your characters, however, if it was the medium weight monocle, you will only be able to use it on medium-armour classes.