Can a row be formed at the back while there is space in the front row?

Observing the order and arrangement of rows in congregational prayers is among the etiquettes of prayer. The imam should show the necessary sensitivity in this matter and, when needed, warn the congregation to arrange the rows properly. The Prophet (peace be upon him) paid attention to the rows being straight and close before starting the prayer and warned his companions on various occasions not to leave gaps between the rows. (Bukhari, Adhan, 71-72 [717-719]; Muslim, Salat, 127-128 [436]) Accordingly, in congregational prayers, it is not in accordance with the Sunnah to follow the imam from the back of the mosque while there is space in the front row. However, the prayers of those who follow the imam outside the rows are still valid.

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu