There is an aspect of the history of mercenaries that is rarely mentioned. States where sometimes leasing their regular soldiers-citizens as mercenaries to foreign states in order to fill their empty state coffins.
For example in 351 BC the Greek city-state of Thebes sent 1000 citizen-hoplites to the Persian king Artexerxes who was campaigning in the rebellious Egypt. Artexerxes payed 300 silver talants (a huge amount) and the Thebans used the money to continue their own war against Athens, Sparta and Fockis. Also, you may have noticed that when you start an EB campaign with KH, a spartan FM (one of the two spartan kings) is in Crete. Historically, he and his army were there not to conquer the island but as mercenaries fighting for aCretan faction to earn some money for Sparta. These kind of mercenaries where actually of sort of patriots.
There is a good book about Greek mercenaries in Classical, not Hellenistic Period "Greek Mercenaries: From the Late Archaic Period to Alexander" by Matthew Trundle. In Google Book Search the first 80 are available to preview which includes chapter 2 titled "What motivated Greek mecenary service"!

What were the big issues in the internal politics of nations other than Rome in the Hellenistic period?
The Hellenistic states where monarchies, so there weren't any politics apart from court intrigues and conspiracies. I'm sure in Carthage there were issues in internal politics but I don't know much about it. Is there anyone who can give some information about it?