Is it permissible to repeat after the Muezzin using more than one formulation, or must one stick to a single formulation?

Following the mu'adhdhin is a Sunnah (recommended act) and not obligatory according to the majority of scholars. The prescribed way is for the listener to repeat what the mu'adhdhin says, except during the two hay'alatayn (when the mu'adhdhin says "Hayya ‘ala al-Salah" and "Hayya ‘ala al-Falah"), in which case the listener should say: "La hawla wa la quwwata illa billah" (There is no might or power except with Allah), and this is the preferred opinion. It is permissible to suffice with saying "wa ana" (and I) or "wa ana ashhad" (and I bear witness) upon hearing the mu'adhdhin's tashahhud (declaration of faith), but it is better to utter the complete phrase. It is recommended for the listener to say, after the mu'adhdhin finishes the two Shahadahs (declarations of faith): "Wa ana ashhadu an la ilaha illa Allah wa anna Muhammadan rasul Allah, raditu billahi Rabban, wa bi Muhammadin rasulan, wa bil Islami dina" (And I bear witness that there is no god but Allah and that Muhammad is the Messenger of Allah; I am content with Allah as my Lord, with Muhammad as a Messenger, and with Islam as a religion).

Al-Nawawi mentioned in Al-Adhkar that the listener says the same as the mu'adhdhin except during the two hay'alatayn, when one says: "La hawla wa la quwwata illa billah." And when the mu'adhdhin says "As-salatu khayrun min an-nawm" (Prayer is better than sleep), one says: "Sadaqta wa bararta" (You have spoken truthfully and righteously) or "Sadaqa Rasulullah, sallallahu alayhi wa sallam, as-salatu khayrun min an-nawm" (The Messenger of Allah, peace and blessings be upon him, has spoken truthfully; prayer is better than sleep). And during the Iqamah, when the mu'adhdhin says "Qad qamati as-Salah" (The prayer has been established), one says: "Aqamaha Allahu wa adama-ha" (May Allah establish it and make it lasting).

After the Adhan is completed, one should send prayers and salutations upon the Prophet, peace and blessings be upon him, then say: "Allahumma Rabba hadhihi ad-da'wati at-tammah..." (O Allah, Lord of this perfect call...), then supplicate with whatever one wishes.