If there is someone who is not married. However, when he gets an Aliyah he usually wears a tallis (follows Ashkenazi Minhagim), should he wear one if called up for an Aliyah at a Chabad where the custom is to never wear a Tallis before marriage or for Aliyah’s in general?
Yes, you should follow your own minhag.
That is to comply with the requirement of not abandoning the Torah of your Mother like is found in the She'iltot d'Rav Achai Gaon 67:2, Tur Orach Chaim 455:1, Chayei Adam, Shabbat and Festivals 127:1, Kol Bo 48:110.
This is also the view of Chabad as explained by the Lubavitcher Rebbe himself in Torat Menachem Hitvadiyut, Vol.8, pp.31-32, beginning with the words, "משתדלים לקיים ולחזק את המנהגים העתיקים".