Benign breast biopsy results.dr said that it was concordant and fellow radiologists looked at it and agreed. is this sufficient as a "second opinion"?

Typically yes: Assuming the second radiologist viewed the images and biopsy results objectively, that could certainly be viewed as a second opinion. On the other hand, some patient's feel strongly that a second opinion should be from a different unaffiliated institution.

It depends: what you are referring to. It is not a second opinion on the pathology results, which would require another pathologist. It does sound like a quasi second opinion on the radiology-pathology correlation, however a true second opinion would require an independent radiologist , not a colleague in the same practice. Why do you think a second opinion is needed?