Reciting a surah after al-Fatihah in prayer is not obligatory, either in an obligatory prayer or a supererogatory prayer, whether the prayer is one in which Quran is recited out loud or one in which it is recited silently, whether one is joining the prayer late or otherwise.

It is permissible to recite part of a long surah in prayer instead of reciting a complete short surah, but it is better to recite a complete surah in each rak`ah. This is what the Prophet (peace and blessings be upon him) usually did.

There is nothing wrong with repeating the same Surah in the Tarawih prayer, or in any other prayer. A person may recite a Surah in the first Rak`ah and repeat the same Surah in the second.

If you are reciting the Surah after Al-Fatihah from the beginning, then you should say the Basmalah (Bismillah ir-Rahman ir-Rahim) apart from Surah At-Tawbah. If you are reciting from the middle of a Surah, then it is not recommended to recite the Basmalah.