The elder apostles of George Balanchine often gripe that his ballets, especially when staged by smaller companies, fall short of the standards of his day. Some directors slow tempos, make minor adjustments to steps or phrasing, or take other liberties with the style. Yet in a March performance, “Twin Masterpieces,” American Contemporary Ballet, located in downtown Los Angeles, displayed exquisite taste in the execution of the works of the “Fathe…