Yes you are. Redistribute five units on an island = stronger mobile defenses, while still maintaining a full garrison when the script kicks in.
Syracuse is a
very early target for both Carthage and Rome players - it's rich and it's close. It's
highly unlikely that the situation you are describing will happen before the player obliterates them entirely.
Rebel recruitment works fine. On two occassions, I was attacked by a combined force of the city I was sieging and the nearby rover army (very poor scouting on my part). I managed to beat them and both the defenders and the rovers were completely destroyed. Not wanting to be
that guy, I didn't take the empty cities. They recruited fresh troops as soon as it was the rebel turn