Is it possible to install rectangular undereave vents or continuous soffit vents in a tongue & groove soffit?

No, because the tongue & groove soffit panels are supposed to lock in place and cutting the vents into the soffit would compromise that locking mechanism.

No, because the tongue & groove soffit panels are supposed to lock in place and cutting the vents into the soffit would compromise that locking mechanism. Even if the locking portion of the tongue & groove was avoided cutting the vents into the soffit would cause the soffit to sag potentially.