Paralus wrote:
Yes, I'll go with vassal state. The individuals mentioned were, as Marcus says, most probably related to the royal family and were thus incorporated as were like individuals in the cantons. Either way, it is likely irrecoverable now.
This leaves open the possibility, these molossians who found themselves in Alexander's army to be 'hostages' sent from the Molossian court during Philip's reign. Alexander of Epirus was one of them. With the exception of Arrybas, the rest are more or less at the same age. I do believe though, there are more than we believe. A certain Admetus is depicted as Macedonian but i strongly believe he is one of this Molossian clan. His name is too common in the Molossian royal house and uncommon in Macedonia.
No matter the input that Olympias may or may not have had, Alexander of Epirus was Philip's choice. He strikes me as a bloke who suffered neither fools nor indecision. Indeed, what does come across is a scheming mind liberally laced with the lifeblood of great leaders: perspicacity. Time and again Greek politicians were undone by this bloke who read them as they read speeches in the Athenian law courts: for a living. It is unlikely, though not impossible, that he knowingly placed on the throne of his vassal state an individual whose "not too fond" view of his patron would see him prepared to attack him one day.
I dont challenge that Philip had these characteristics - he certainly did - but in the specific case of choosing the head of Epirotic kingdom his alternatives are almost minimal. The candidate should be a member of the royal molossian house for obvious reasons, not a follower of Arrybas and at the time being, Olympias grabbed the opportunity to promote to Philip her young brother as the best option.
Peter Green in his 'Alexander of Macedon' p. 97 goes further to analyze it a little more.
"
Alexander of Epirus was an independent and ambitious youth. The fact that he owed his throne to Philip weighed not at all with him; he probably regarded this as no more than a fair returnfor having to put up with his brother-in-law's homosexual attentions at an impressionable age"
All that which Olympias may well have done, although having its genesis in hatred of Philip, was conceived and carried to put her son on the throne and, so too, return the fearful old Queen Mother back to her rightful position. That she would have countenanced civil war against Philip is, on the probabilities, very unlikely. It will have set many of those who backed Alexander, after Philip's convenient murder, against him. Philip had spent a lifetime bonding commanders such as Cleitus, Parmenion, Antipater, Polyperchon and, most recently, Attalus to him. They will have sided with their King. Alexander was, at this stage, no great popular demi-god.
At the time when Olympias endeavoured to induce her brother Alexander to make war on Philip, she was almost in a dead end. She and her son were in exile, another woman was at her place as queen of Macedon and her unborn child posed to be a serious threat to the succession in the throne of Macedonia. The incident where her son Alexander went to Illyria, instead of Epirus as normally he should, brings a question. What was Alexander doing there?
From a skeptic's point of view this could mean easily the research of allies among Illyrians against Philip. The Macedonian army was far superior to the forces of Epirus but a possible coalition with Illyrians meant two open fronts for Philip and better chances for Olympias and Alexander.(Note that i dont claim if this was the case they would defeat Philip. Macedonian army had no match at that time.)
The lack of Epirote forces with the invasion force is intriguing (I assume you are referring to Diodorus' list). There might be any number of reasons. Alexander took only 7,000 Greek forces of the League of Corinth with him. These, for the most part, went along sight seeing (as Archange might put it). Yes, overstated but they were – at best – severly under-utilised and dispensed with or recommissioned as mercenaries as soon as practicable. Again, this was a Macedonian exercise with Greek bedclothes.
In total he had about 14,400 Allied Greeks (counted inside all the greeks nomatter mercenaries or from the League of Corinth)
James R. Ashley gives these numbers for Greeks.
Cavalry
2,400 (from Thessalia, Orchomenus, etc)
Infantry
7,000 Greek allies (league of Corinth)
5,000 Greek mercenaries
and i add the 160 triremes of the allied Greek navy.
He did take (using Bosworth's figures) some 15,000 Macedonian foot and left 5,000 at home with Antipater (Bosworth's reading of the text and the corrupted figures is, I think, correct). Did Epirote forces ever serve Macedonian armies as mercenary contingents? Philip often used mercenaries – in the order of thousands – but they are never identified. Surely most were Greek. Antipater's forces will have been little different.
More reading.
I dont think Epirote forces ever served as mercenaries in Macedonian armies but the opposite certainly became true in Hellenistic ages under Pyrrhus.

There are maybe 1-2 cases in classic ages where Macedonians and Molossians made coalitions to counter the numerous Illyrian incursions in their territories. Generally Epirote werent accustomed to be mercenaries.