Yup. For an unit to be recruitable as a merc in RTW, it needs to be given the attribute "mercenary" in the EDU; from which point onwards it then automatically uses the model's "merc" skin slot as defined in the DMB, irrespective of whether it has been recruited from the merc pools or barracks and the faction doing the recruiting. (This is also used to increase the number of units that can be put under the same model for a given faction.)