What is the Fiqh (Islamic jurisprudence) regarding how to deal with those who chant poems containing polytheistic (shirk) words? And should they be debated, given the limited knowledge and fear of slandering the Salafi creed or spreading animosity?

You must offer sincere advice with wisdom and gentleness to those who work with you, especially your employer. Seek help from Allah, and then from anyone who has a connection to your employer, if that person has knowledge of the creed of Ahlus-Sunnah. Do not undertake discussions with them if you do not have sufficient knowledge; rather, inform knowledgeable callers to Islam about their situation so that they may remove their misconceptions. If advice does not benefit, then denounce the evil to the best of your ability. If you are unable, then do so with your heart, which is the weakest of faith. If you can avoid their gatherings where evil is present, then do so, in compliance with the words of Allah Almighty: "And when you see those who engage in [blasphemous] discourse concerning Our verses, then turn away from them until they enter into another conversation. And if Satan should cause you to forget, then do not remain after the remembrance with the wrongdoing people."