Is it obligatory for one who is capable of deriving rulings from the evidence to avoid adhering to a specific madhhab (school of thought), and is one who seeks a fatwa from someone who possesses this capability yet adheres to a madhhab sinful?

If a mukallaf (accountable person) reaches the rank of ijtihad (independent juristic reasoning), it is not permissible for him to imitate (taqlid) any other mujtahid. As for the non-mujtahid who adheres to the madhhab (school of thought) of a specific imam, he may issue a fatwa (religious ruling) based on the قول (saying/opinion) of that imam by way of taqlid, when there is a need and no mujtahid scholar is available. Therefore, there is no harm in seeking a fatwa from a Shafi'i professor, as he will issue a fatwa based on his ijtihad if he is a mujtahid on the issue; otherwise, he will issue a fatwa according to the madhhab, and this is permissible out of necessity. Adhering to a madhhab is permissible, provided there is no fanaticism. If the truth becomes apparent to the scholar in disagreement with his madhhab, he should revert to it, as did great scholars such as Ibn Abd al-Barr, al-Nawawi, and Ibn Taymiyyah.