Is it permissible for someone who finds a congregation praying before the time of the mosque's main congregation to pray with them, knowing that the main congregation is larger in number, and is it obligatory to interrupt a regular voluntary prayer (ratibah) to join them?

There is no harm in multiple congregations in a prayer area (musalla) that does not have the ruling of a mosque. Each congregation has its reward, and the best of them is the one with the largest number. It is not necessary to cut short a supererogatory prayer if the congregational prayer is established, as long as one does not fear missing the congregation, due to the Almighty's saying: "And do not invalidate your deeds."