What is the ruling on an individual praying alone in the mosque when another congregation is praying? And is it permissible to interrupt one's prayer to join an ongoing congregation?

Your prayer performed individually, even if a congregation is present, is valid, though praying with the congregation is superior due to its immense reward. Congregational prayer is not a condition for the validity of prayer according to the majority of scholars. If you catch the last tashahhud (definition) with the Imam, you have indeed caught the congregational prayer and are not considered to have prayed individually. In such a case, it is not permissible for you to break your prayer to join another congregation.