Is it permissible to lead people in prayer during Fajr and Isha, and what are the requirements for the preacher in the Friday prayer?

Fajr and Isha prayers, like other prayers, are legislated to be performed in congregation. Indeed, it is an individual obligation (fard ayn) upon free, adult men who have no valid excuses. Leading the prayer (Imamah) in them is legislated, and there is no harm in a person leading people in some prayers but not others. As for the conditions of one who delivers the Friday sermon and the pillars of the Friday sermon, they have been explained previously in earlier fatwas.