Eight vidyadharas

Eight vidyadharas (Tib. rig 'dzin brgyad) - Vimalamitra, Humkara, Mañjushrimitra, Nagarjuna, Padmasambhava, Dhanasamskrita, Rambuguhya and Shantigarbha.

In which circumstances is this correct, and in which not? For example, in relation to Vajrakilaya, the eight vidyadharas include Prabahasti, not Padmasambhava. Padmasambhava received a ninth casket from Chemchok Heruka at a later time.