Where would Ptolemaic Theron go?
Somewhere on the coast in the South of modern Sudan / North of modern Eritrea.
Was there a big ivory trade in the area?
Meroe was responsible for exporting ivory from the Butana and further South. In Ptolemais Theron there might have been some ivory trade aswell, but in that area they needed the elephants to stay alive. Only source for ivory would have been rhino horns.
Also, do you know anything about the blemmyes tribes?
Blemmyes were not really relevant during this timeframe. Why? Do you want a Blemmye unit?
Also a few mods depict Ethiopian hoplites as if some peoples were hellenized. Any truth to that?
Very likely fantasy. Ptolemais Theorn for example would have consisted of Greeks and Egyptians. The local Africans were mostly nomads and its highly unlikely that they were Hellenized in such a degree that they would start to fight as hoplites. I would suggest ordinary Greek units instead.