Suppose 2 capacitors are connected in series, the plates connected to the battery terminals receive charges $+q$ and $-q$, and the isolated plates in the combination receive equal and opposite charges through induction. Now my question is, why is that the induced opposite charge on the plates isolated (i.e. not connected to the rest of the circuit through wires as in any ordinary series combination of capacitors) from the other two plates equal to the inducing charge on the plates connected to the battery?
A capacitor is a storage element for electrical field energy. As such, its electric field is internal to the element; that is, field that originates on one of the capacitor's plates terminates on the other plate. That condition is enough to guarantee that the charges on a capacitor's plates are always equal in magnitude and opposite in sign.