In my opinion, the Triarii do not need to be individually recruit able : they are already represented inside the « Legion » unit. The legion’s line infantry of the republican era is made of Triarii, Hastati, and Principes. In FOGE, the « Legion » unit represent all of these units.
If you export a battle to Field Of Glory 2 playing as Roma, you’ll see that your FOGE « Legion » unit is converted to « Hastati/Principes » and to « Triarii » FOG2 units
Making the Triarii individually recruitable in FOGE would be redundant