The time for Tarawih begins when the ‘Isha prayer is over. There is nothing wrong with praying ‘Isha behind one who is praying Tarawih.

Standing in the row during the congregational prayer is obligatory. If a person comes and the row is full, he should pray with the congregation behind the row, and not go forward to stand beside the Imam or pull someone out to stand beside him, nor should he abandon the prayer in congregation.

Two people are enough to pray in congregation, whether that is in the house or elsewhere. However, men are required to offer the prayers in congregation in the mosque. It is not permissible to offer the obligatory prayers at home, whether in congregation or alone, unless one has an excuse.