It seems to me that, while being closely tied because they are both human settlements, Choras is a lot more distant both geographically and politically from Guardia than Porre is. That is, Choras is likely to be its own state, and not merely an overseas province.
I could not pretend to prove what may have caused Choras's downfall, but Choras seems to have been a grander kingdom at some point. Since its power dwindled, however, it seems to have turned more into a simple communal society - a commonwealth is probably the closest terms. This is evidenced by an Elder, the figure of which seems to be supreme in most other instances within the game (prehistoric tribes, domes in the future), whereas Porre is limited to a Mayor, suggesting a superior hierarchy (King Guardia, more specifically), and also as a lack of references to being under Guardian control, whereas in Porre we have enough to suggest that Porre is, indeed, some sort of Guardian province.
As to the question of Cyrus, it seems to me that Choras, appearing to have lost its military and political might already in 600 A.D., would have trusted all its able-bodied fighters to the guidance of the stronger Kingdom of Guardia, and as such any of its young men would have been shipped off to be trained there. But, accepting this as the likely situation, it would not seem likely that Glenn came from Choras; as Cyrus puts it, he's a marshmallow, and it's doubtful he would have been considered useful as a soldier and never have been thought worthy of being enlisted from Choras overseas. But Cyrus seems to be popular in general - probably the best foreign enlistee in the army - and as such would earn quite a few young admirers. Glenn may have been closer to Cyrus than this, but fanboy is probably the best term for him nonetheless.
This is just what I always thought, anyhow - there's little proof for any situation. The only reason I bothered to mention these thoughts is because this is the way my world is organized in Into the Dream, and I thought I'd back it up as best I could. I was too far into it before I realized that the only way to back up any of these positions is, really, personal preference.
EDIT: Oh yeah. Wouldn't any fight against the Mystics, really, be a fight for humans in general? I mean, even fighting under the army of Guardia, any soldier joining the struggle against Magus would really be working to save both Guardia and Choras - you know that generic "save all of humanity" thing.