Whilst i know that the council of Nikhae did ruled against the use of warp magic. But am i correct in thinking that there was a group of librarians, from various legions who spoke at the council.

Am i correct in thinking that this suggests that librarians were in use in the legions before this time, and as a result would be appropriate for use before hand?

i know that typhon of the death guard was a psyker but kept it hidden as mortarion hated them, and i do recall reading somewhere that a few legions did employ the use of librarians (but i cant confirm this)

but i would be inclined to say yes

hopefully someone with a better memory than me can give you a proper answer :D

I was under the impression that the council of Nikea ruled out the use of sorcery, not librarians.

The Librarians obviously existed to record and such for the legion, and the role as a psychic would have been secondary.