Is my prayer valid, or must I repeat it as two or four rak'ahs, if I prayed 'Isha shortened (two rak'ahs) behind a congregation praying Maghrib, and I joined them in the second rak'ah, and I said the taslim with the Imam?

If a traveler prays behind a resident Imam: If the prayers are of the same type, then the traveler must complete the full prayer. If they are different, such as a traveler praying 'Isha behind a resident who is praying Maghrib, then the traveler has the choice to either pray two rak'ahs or to complete his prayer to four rak'ahs after the Imam says the taslim. His prayer is valid in both cases.