It is not Sunnah to recite Qunut in Fajr prayer all the time. Your prayer behind an Imam who recites Qunut in Fajr is valid, but if there is someone who does not always recite Qunut in Fajr, it is better to pray behind him, so as to follow the Sunnah.

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.