Is it permissible for me to pray 'Isha behind someone who is praying the Rawatib (supererogatory) prayer without the intention of leading, and is it permissible for me to join him while he is bowing (in ruku') to catch the rak'ah?

It is permissible for a person performing an obligatory prayer to pray behind someone performing a supererogatory (nafl) prayer. This is based on what was narrated about Mu'adh ibn Jabal, who would pray the 'Isha (night) prayer with the Messenger of Allah, peace and blessings be upon him, and then return to lead his people in the same prayer. For him, it would be a supererogatory prayer, and for them, an obligatory prayer. The Maliki and Shafi'i schools of thought hold that the absence of the intention to lead at the beginning of the prayer does not affect its validity. It is permissible to join the prayer with the imam in any state, based on the saying of the Prophet, peace and blessings be upon him: "If one of you comes to the prayer and the imam is in a certain state, let him do as the imam does."