Is it permissible to pray in a congregation (Salat al-Jama'ah) before the main congregational prayer (Iqamat al-Jama'ah al-Kubra) because it clashes with a university lecture?

The jurists differed regarding the ruling on congregational prayer in the mosque. The majority of jurists held that it is a Sunnah and not an individual obligation (fard ‘ayn), but rather a collective obligation (fard kifayah). Imam Ahmad, in one narration, held it to be obligatory in the mosque if the mosque is nearby.

As for the ruling on congregational prayer in general (whether in the mosque or elsewhere), the Malikis and Shafi’is held it to be a confirmed Sunnah (Sunnah Mu’akkadah). The Hanafis and Hanbalis held it to be obligatory.

The most preponderant view is the obligation of congregational prayer, except for one with an excuse, whether it is in the mosque or elsewhere, with performing it in the mosque being better. Based on this, your congregational prayer before the mosque's congregation is valid, and you will receive the reward of the congregation for it.