If I begin reciting with a six-vowel-count madd, must I complete the entire surah with that same count, or is it permissible for me to shorten the madd to two vowel counts and finish the surah with that? Which of the two is correct?

Al-Madd (lengthening) is the prolongation of the sound when pronouncing a Madd letter. It is divided into Asli (natural) and Far'i (secondary). Al-Madd Al-Lazim occurs when a lazim (compulsory) sukun (vowellessness) follows a Madd letter, whether in continuous recitation or when stopping, and whether it is in a word or a letter, and whether it is mukhaffaf (lightened) or muthaqqal (heavy). The ruling for Al-Madd Al-Lazim is that it must always be prolonged for six harakat (vowel counts) by consensus, with the exception of the word (‘Ayn) in (Kaf Ha Ya 'Ayn Sad) and ('Ayn Sin Qaf), where two recitations are permissible: al-ishba' (six harakat) and al-tawassut (four harakat). Shortening Al-Madd Al-Lazim is never permissible under any circumstances.