I can only tell, that the formulas in ISO 6336-3 are different for internal and external gears. For internal gears a rim thickness larger 3.5 times module is required to avoid loss of strength, for external gears it would be a rim thickness of 1.2 times whole depth.
The formulas are YB = 1.6 * ln(2.242*ht/sr) for external gears and YB = 1.15*ln(8.324*mn/sr) for internal gears, with YB > 1.
So ISO 6335 requires a larger rim thickness for internal gears as for external gears as long the whole depth is smaller 2.9 times module, which is usually the case.
I don't know how they compare to AGMA 2001, I just remember AGMA has only the drawing for an external gear in there.