This is a very peculiar form of Guru Mandala where we start the puja starting from Kameshwara and KAmeshwari - the Adhi guru couple and end with our own Guru, Parama Guru and Parameshti Guru.
Shambhava padhukaa is not necessary to do this yajanam. But Poornabishekam is very much important for this. This is more or less like a Sanyasi's "Vyasa Puja".. This differs among Guru paramparas. So it is better to take advise from Swa Guru before doing this yajanam. Pournami, Guru parvas, Guru Poornima, etc are very auspicios days. Guru Pournami is very very auspicious for doing this yajanam.