umm, as Arkhe Seleukeia and as Ptolemaioi, when I build a level 4 regional barracks in Nikaia, I can recruit Misthophoroi Thraikioi Prodromoi and Misthophoroi Thraikioi Peltastai, and the units have the "merc" designation in the recruit window (they are named the full name that I typed out). I don't know exactly why they did it like that, though it could be that there are hardcoded separate slots for "factional" units and "merc" units (I think I did read this in an old post here).