Bismillah Alrahman Alraheem,
I am sure that you are already aware that much has been written by Twelver scholars about the reliance of Sunnis on Shia narrators. Of course, you are most likely also aware that the typical Sunni response is that there are no evidences that these Shias are Twelvers, but rather, are political Shias, that preferred Ali to Uthman, or were those that were critical of the people of Al-Shaam. However, neither of the two sides, in my opinion, have brought sufficient evidence to back up either claim, even though the burden of proof is upon the Twelvers of today.
In this post, I share a few examples that will be sufficient in proving that there is no reason to believe that when Sunni scholars used terms like “shi’ee” or “rafidhee” that they were not referring to Twelvers.
1) Mohammed bin Ishaaq Al-Madani, the author of the seerah:
The Sunni scholar Al-Khateeb Al-Baghdaadi said: Some scholars didn’t see authority in the narrations of Ibn Ishaaq for reasons like: his tashayyu, his attribution to al-qadr, and his tadlees.
The Shi’ee scholar Al-Tusi said under his list of companions of Al-Baqir: Mohammed bin Ishaaq Al-Madani,sahib al-siyar, ‘aami.
2) Abdulsalam bin Salih Al-Harawi:
The Sunni scholars said:
Yahya bin Ma’een said: Thiqa saduq ila anahu yatashayya’.
Al-Daraqutni and Al-Uqaili said: He was a vile rafidhi.
The Shia scholar Al-Tusi said: Aami.
3) Abbad bin Ya’qoub Al-Rawajini:
The Sunni scholars said:
Ibn Adi said: He is extreme in his tashayyu.
Salih bin Mohammed said: He used to curse Uthman.
Al-Daraqutni said: He is a truthful shi’ee.
Ibn Hibban said: He was a rafidhi that called to it.
The Shi’ee scholar Al-Tusi said in his Fihrist: He was Aami al-mathhab.
4) Khalid bin Tahman
The Sunni scholar Abu Hatim said: He was one of the old shia.
The Shia scholar Al-Najashi said: He was one of the aama.
5) Ammar Al-Duhani:
The Sunni scholar Sufyan bin Uyayna accused him of tashayyu.
The Shia scholar Al-Najashi said about him, in the biography of Ammar’s son: His father Ammar was a thiqaamong the aama wajhan (a major representative of them).
Note: The term aama is the term used by Shias to refer to Sunnis.
In conclusion, as we can see there is a clear contradiction between the opinions of the Sunni and Shia scholars. So, the only way to reconcile these contradictions is by saying that these narrators are Shi’ee in the eyes of Sunni scholars, in the sense of their political outlook, but are Sunnis in the eyes of the Shia scholars, for they were not Twelvers.