Salaam. Bismillah AlRahman AlRaheem.
"If two parties of believers fight against each other, make peace between them; then if after that one of them transgresses against the other, fight the party that transgresses until it submits to the command of God. Then if it complies, make peace between them with equity, and act justly. Truly, God loves the just" Surat AlHujurat Ayat 9
Only Allah chooses the succesor. To say one is sunni, shia or Sufi is to cause division. To follow a madhab meaning division, is to cause separation. To believe and Love Allah and to follow the Qur'an and habits of the RasoolAllah is to be a Muslim.
A wise person once told me, if we take from the "Sufi" thier intense Love for Allah AlRahman AlRaheem and our beloved RasoolAllah, and the internal struggle to pay mind to our intention, to control our anger, lusts, vanity and pride and to take from the "shia" their love for Ahlul Bayt and the habits of our RasoolAllah, and to take from the "sunni" their external love for societal harmony and their strict adherence to the Laws set by Allah with the glorious Qur'an, then that is to be a Muslim.
The separation of the Christians was caused by the writing of Prophet Isas stories hundreds of years after he was gone, which people disagreed on. The changing of the Torah was caused by the collecting of writings (akin to hadith) hundreds of years after Prophet Musa was gone. And the same such has happened to the Muslim.
We can read hadith but Law comes from Qur'an, as it is absolute and complete. Hadiths should all be taken with grains of salt with regard to them being attributed to our Beloved RasoolAllahs habits. We must use our sense when reading them and know that nothing can be added, ammended or taken away from the glorious Qur'an, and judge accordingly. Hadiths can be read for historical purpose as well, as we have no solid evidence of the events of the times after our RasoolAllah was gone, but again with grains of salt.
The division of Muslims is a political agenda and always has been. We must grab to the rope firmly and become one again, inshaAllah. With regards to who was to "lead" after our RasoolAllaha death is a matter that we still waste time trying to resolve hundred of hers later causing Sunni and shia violence towards one another. We should leave political matters behind and have Love for those our RasoolAllah loved, both Ali may peace and blessing be upon him and Abu Bakr may peace and blessings be upon him. Unless we can find original proofs on their original papers, then we can never what truly happened to cause such a division. To point out, Shia in the Glorious Quran means "followers".
Take knowledge from all those who give it and judge accordingly, and with Allahs leave we will always see Truth from Falsehood.
If what We seek is Truth, Allah never goes back on promises.
Allah Knows best and in Allah do we put complete trust.