Should I pray with the congregation or individually and shortened, if I have prayed Maghrib with the intention of combining and shortening, then I find the Iqamah for 'Isha prayer in the mosque?

If a traveler joins a congregation in a mosque, he should pray the full prayer (itmām) and not shorten it (qaṣr), because shortening the prayer is a Sunnah (definition), while the virtue of congregational prayer is more emphasized. Malik disliked for a traveler to pray behind a resident (muqīm) except for certain reasons, among which is his presence for prayer in the grand mosques of cities and major capitals.