There are three things you have to do to make a unit playable.

1- Make sure they have a skin for their faction for the correct model (in the descr_model_battle.txt). The Romani (internal name= "seleucid") already have a skin for the Syracusan Hoplitai, for some reason, but they don't have one for the Massilioi Hoplitai. You'll have to copy another faction's lines for skin and sprites and then change their internal name to "seleucid". (If the faction is already unit that particular model for a different unit, it will be impossible for you to get the new unit.)

2- Ownership in the export_descr_units.txt. Just add the internal name of your faction to the ownership line for that unit. You have this under control.

3- Recruitment in the export_descr_buildings. The easiest way to do this is find the recruitment line for factions that can recruit the unit and copy it. Then find the MIC area for your own faction and paste it and change the internal name to your faction. (Remember to apply this to all the MIC levels after the level you first want them to be recruitable on.) (Be careful, you can easily get lost or make mistakes in the EDB.)

4- This isn't needed to make the unit work, but you'll probably want to copy one of the UI pictures from a faction's folder that can recruit the unit to your faction's folders. (Remember both the UNITS and UNITS_INFO folders.)