We can manipulate the diplomacy section for that then. I've actually started on it already.
There are two values, one dictates the relationship between factions at the start of the game. The other value, the core value, tells us how the faction feels towards the other faction. Thus, if two factions start at war but have positive core values, then later on in the game they are likely to become good allies. Conversely, if two factions start off allied but are hostile to one another at core, then there is a very good chance they'll end up at war with one another.
Epirus and Macedon are going to be rather hostile to one another with their core values. Not sure they should start at war though? Has Pyrrhus had any scraps with Gonatas before his trip to Italy? Since it is his dynasty that we are representing with Macedon in the game.
Epirus is likely to be allied with the Greek Cities at the start, but their core values are going to be so-so, going on to hostile. Bear in mind that the Greek Cities represent Keraunus, the Aetolians and Syracuse which are all somewhat on good terms with Epirus near the beginning few turns (for whatever reason). Epirus will turn on the greeks if they have an opportunity to or if the situation is forced on them.
Epirus and Romans start at war, but the core value is much lower and they are otherwise so-so with each other. Pyrrhus wasn't all that interested in conquering Italy and didn't see the Romans as the real threat to the Greek world. The Romans are still very likely to force a fight with them, but Epirus won't be as inclined to return the favour. Except from the beginning.
Macedon and the Seleucids can start at war, but their core values are rather good. Since the AI will readily accept a ceasefire if the two factions do not share a border, they should very quickly patch up their relationship.