What is the ruling on acting upon juristic opinions that are lenient and contrary to the majority and to some of the Four Imams, and which are adopted by Sheikh al-Islam Ibn Taymiyyah and al-Albani?

Acting upon the opinions of the two Sheikhs, Ibn Taymiyyah and Al-Albani, is permissible, as both are among the mujtahid imams. As for the blameworthy following of concessions (rukhsahs) in Islamic law, it refers to adopting their opinion when it is a concession, and then moving to another scholar's opinion if it offers a different concession. However, if one adopts their views because of their known rigorous investigation and adherence to evidence, along with the breadth and ease found in their opinions, then that is permissible for you.