Here are a few possibilities:

Engyum - Half way between Henna and Halaesa, but west of both.

Motyum - West of Henna, and half way between Henna and Akragas.

Inycum - North west of Akragas.

Paropus - South of Himera and Thermae, and east of Hippana.

How many cities are you looking to add?

Concerning Agrigentum, if the campaign is pre-Roman, the name should be Akragas. It's name was changed (latinized?) to Agrigentum after it became a Roman colony, so if you make a 400's BC campaign, I would call it Akragas.

If I can do anything else, just let me know.