Is it permissible to say "may Allah have mercy on them" (al-tarahhum) or "may Allah forgive them" (al-istighfar) for the Companions instead of "may Allah be pleased with them" (al-taraddi 'anhum)? And what is the ruling on one who does so, citing the noble verse: {Our Lord, forgive us and our brothers who preceded us in faith}?

There is no harm in saying "may Allah have mercy on him" (رحمه الله) or "may Allah forgive him" (غفر الله له) concerning a Companion. This is supported by Allah's saying: "Our Lord, forgive us and our brethren who preceded us in faith," and the Prophet's prayer for the people of Al-Baqi', where he said: "O Allah, forgive the Ansar and the Muhajira." However, it is not permissible for this to be said in a manner of disparagement or contempt. The most preferred and better expression is to say: "May Allah be pleased with him" (رضي الله عنه).