Is it permissible for one to sometimes pray as a resident (without combining or shortening prayers) and at other times combine and shorten prayers, or must one adhere to a single opinion?

Shortening the prayer for a traveler is a confirmed Sunnah, whereas congregational prayer is obligatory upon him. If the Imam is a resident, the traveler should complete the prayer behind him. Combining two prayers is permissible for a traveler, but it is better not to combine them unless there is hardship in not doing so, as combining is not a confirmed Sunnah like shortening. Thus, the traveler does not have a choice regarding shortening the prayer; rather, shortening the prayer is confirmed for him unless he prays behind a resident Imam, while he does have a choice regarding combining prayers.