The group Zaidees also believe that the Sahabahs took oath on 'Ali's (رضى الله عنه) caliphate? What is Aqeedah difference between Zaidees and mainstream sunnis?
It is said that the Zaidiyyah are the closest of the Shia'ee sects to Ahl-us-Sunnah. However, this ruling only applies to the original followers of Zaid and the early Zaidiyyah. Later on, many leaders among them appeared and distorted the group’s original teachings until some of the sub-sects became closer to the Raafidah Imaameeyah!
The Zaidees split up into at least eight sects, but only three of them are famous:
1- the Jaaroodeeyah, who claimed that the Prophet (ﷺ) appointed ‘Alee (رضى الله عنه) as his immediate successor by description, not by name, and that the Companions deviated and disbelieved by failing to follow him after the Prophet’s death!
2- the Sulaimaaneeyah, whose founder, along with some other lesser known Zaidee sects, declared ‘Uthmaan (رضى الله عنه) a disbeliever because of the (false) accusations levelled against him!
3- the Butreeyah, whose founder disowned ‘Uthmaan (رضى الله عنه) because of the (false) accusations levelled against him.
Therefore, the first two of the three well known Zaidee sects are in fact Raafidees, not close to Ahl-us-Sunnah at all. As for their ‘aqeedah concerning Tawheed-ul-Asmaa’ was-Sifaat and matters of emaan, most of the later Zaidees in general follow or incline towards the teachings of the Mu‘tazilah.
And Allaah knows best. 
- Maqaalaat-ul-Islaameeyeen (1/141-145), by Abul-Hasan al-Ash‘aree.
- Firaq Mu‘aasirah Tantasib ilal-Islaam (1/335-336 & 343), by Dr. Ghaalib ‘Awaajee.