I was wondering if the to hit, the to wound, and the armour saves rolls are all related like this:
A nob with squigs is hit. The nobs are in the majority and so they take the hits and wounds and then the nobs who are wounded get to take an armour save in order to avoid becoming casualties.
or
A nob with squigs is hit. The nobs are in the majority and so they take the hits and wounds but they and the squigs have the same armour value. Even though it was the WS and T of the nob that you rolled against can you allocate the attack to a squig and if the save is failed remove the squig as a casualty instead of the nob.
note: squigs are part of the unit, as in their own model, much like chaos hounds, kroot hounds and ferisian wolves.