Abu Hanifa al-Nu'man ibn Thabit; Anas ibn Malik; Muhammad ibn Idris al-Shafi'i and Ahmad ibn Hanbal. The first two Sunni Imams studied under Imam Ja'far al-Sadiq, the sixth Imam of the Shi'a.

In general, they rely upon the narrations of companions that are rejected by the Imams of the Shi'a owing to the fact that these companions actively opposed the fourth caliph, Ali ibn Abi Talib. The Imams of the Twelver Shi'a narrated directly back to the Prophet (s) via Ali ibn Abi Talib.

They rejected the legitimacy of the caliphates of Abu Bakr, 'Umar and 'Uthman. Initially the name was used pejoratively by the 'Uthmaniyya, but it was taken up and used by the Imami Shi'a themselves.

"Shia", in his technical sense, originally indicated a person who followed Ali and preferred him to the other sahabah. It was not supposed to be a sect or a variety of doctrinal branches with different theological or religious views. As a matter of fact divisions were spread, the book "Firaq al-Shia" by Hasan Ibn Musa al-Nawbakhti, available also in English, mentions most of their ramifications. Nowadays the main surviving schools are the Zaydiyyah (Zaydis), the Ismailiyyah (Ismailis) and Ithna Ashariyyah (Twelvers).

With prayers for your success.

It is not prohibited. However some ulama says it is mostahab to make day of Ashura, day of sorrow and do not eat so much during the day. 

You can break your fast immediately at the time of adhan, if you are sure it is the correct time of adhan.