AFAIK in XVIIth century the commonwealth didn't have a solid army and the state army was relatively small, compared to the armies fielded by the landlords. So to model it I came with an idea.
First of all, let's have some units avaible as mercenaries - for example the nobles (szlachta) units should be recruited while outside towns, so that a general would have to make a run through a few provinces to gather an army (to simulate the habit of calling comrades under arms, that usually took some time). In cities you should be able to train more specialised infantary, like piechota wybraniecka etc, though the soldier pool would be way lower than in other factions.
Also, as many armies were fielded by landlords, we can try to show it this way - place some rebel armies in a few forts scattered around the country. They would be passive, so you wouldn't have to fight them. Then, in case of war, or war expedition, you could bribe them, so that they'd join you. They would be quite unloyal -so you would have to keep an eye on them - (maybe, if it's possible, their loyality would go down as the time passes on) and maybe they'd also cost a bit, so that after the war you'd have to disband them and they'd show as rebels back in their fort, ready for next bribing :p With this system, The Commonwealth wouldn't have a huge, constant army, thus its power would be fairly limited - and if we take into account that the Polish/Lithuanian army of this time was one of the best, it would prevent Poland from becoming the superpower too fast.
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