Is it compulsory to say "Allahu Akbar" while praying behind the Imam individually? Or should I just follow the Imam?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Pronouncing Takbeer upon moving from a position to another position is compulsory according the preponderant opinion of Muslim scholars. Therefore, intentionally leaving it is not permitted; whoever intentionally leaves it commits a sin and his prayer becomes void. This ruling applies to an Imam, follower and the one who performs his prayers alone. On the other hand if one leaves the above Takbeer due to forgetfulness he should make two prostrations due to the forgetfulness, and then his prayers become valid. Know that the forgetfulness of a follower is not taken into consideration during a congregational prayer even if he is the only follower of the Imam.
Allah knows best.