I was interested to discuss what the population figures for several Hellenistic states would have been like. Concerning Macedonia, I'm not aware of any sources regarding the population and, considering that the Macedonians raised a native army of 21,000 phalangites, 3,000 hypaspistae, 2,000 heavy cavalry and some 3-4,000 Macedonians as garrisons during the Third Macedonian War, in all some 30,000 native Macedonians (ignoring the mercenaries and the Galatians, Illyrians, Thracians and Greeks who were settled in the Kingdom), I'd say the Macedonian Kingdom had a population of some 300,000 Macedonians and some 50,000 foreigners (those mentioned above).
Concerning Epirus, things are even more difficult, concerning sources. What we know is that the Romans captured and sold as slaves 150,000 Epirotes who were in their majority Molossians, as the Thesprotians and the Chaonians who were the other two Epirote major tribes had sided with Rome during the Third Macedonian War.
About the population of Greeks and Macedonians in the Seleucid Empire, the best source is to consider the figures given for the Battle of Magnesia, namely 16,000 phalangites, 10,000 Argyraspidae and a few thousand cavalry, probably all in all 30,000 Greeks-Macedonians. Of course it is to be expected that such a large dominion would have kept a large number of military settlers in the Upper Satrapies or in the Seleucid-Ptolemaic border a figure I'd estimate to at least 10,000 additional men. I'd estimate the overall Greek-Macedonian population of the Seleucid Empire to about 200-300,000 people. Thoughts?