Is it permissible for the Imaam to stand in the same row with the Ma'mumeen instead of standing alone ahead of the rows due to the narrowness of the mosque?

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

The Sunnah is that the Imaam (i.e. the one leading the prayer) should stand ahead of the rows of the Ma'mumeen (i.e. the ones being led in the prayer) if they were two or more. The Prophet, sallallaahu ‘alayhi wa sallam, used to stand ahead of the Companions in the congregational prayer and they used to stand behind him. Jaabir and Jabbaar stood next to "the Prophet, sallallaahu ‘alayhi wa sallam, in a congregational prayer with one standing to the right side of the Prophet, sallallaahu ‘alayhi wa sallam, and the other to his left. The Prophet, sallallaahu ‘alayhi wa sallam, caught hold of the hands of both of them, pushed them back and made them stand behind him." [Muslim]

Scholars excluded a naked Imaam who is leading a group of naked Ma'mumeen. They believed that it is recommended for him to stand next to the Ma'mumeen in the same row. An-Nawawi said in Al-Majmoo‘, "If a group of naked people has no one among them who is dressed to lead them in the congregational prayer, it is recommended for the Imaam in such a case to stand in the same row with the Ma'mumeen and it is also recommended for the Ma'mumeen to stand in one row so that they would not look at the ‘Awrah of one another. If this is not possible and there has to be two rows, the Ma'mumeen should lower their gazes and perform the prayer anyways."

Hence, if there is enough space in the mosque for the Ma'mumeen to stand in two rows, the Sunnah is that the Imaam should step ahead of the worshippers and they should stand behind him. If it is not possible, he may stand next to them in the same row as the Imaam's standing ahead of the congregation is just an act of the Sunnah and not an obligation. The benefit of gathering to perform the congregational prayer together is greater than that of the Imaam standing ahead of the Ma'mumeen.