Is it permissible to supplicate with "O Allah, grant me a truthful tongue among others, O Allah, grant me a truthful footing with You"? And why was it said "among others" (fī al-ākhirīn) and not "among the former ones" (fī al-awwalīn)? And why was "footing" (qadam) chosen in the second supplication?

These two supplications are excellent, and it is permissible for a Muslim to supplicate with them for himself and for others, because they are derived from the Holy Quran. The first supplication: (My Lord, grant me authority and join me with the righteous; And grant me a reputation of honor among later generations) means requesting a good mention and beautiful praise after death. The second supplication: (And give good tidings to those who believe that they will have a firm footing with their Lord) means that believers will have a good reward for the righteous deeds they have put forth. So, "a reputation of honor" (lisān ṣidq) is praise and good mention among later generations, and "a firm footing" (qadam ṣidq) is a good reward from Allah.