I was wondering wondering if these presumed persian hoplites also made their way into seleucid army(also, if not, what happened to them; it's hard to believe they were compeltely annihilated at Issos, especially since they presumably appear in the armies of many neighboring nations).

According to EB's unit list, they appear in the armies of Hayasdan, Pontos and Parthia but not Seleucids. However, the unit description speaks of their evolution and I've also found some mentions of 'cardaces' in historical descriptions of seleucid armies.