If you're looking for manga similar to D4DJ First Mix, you might like these titles.
The "idol-training produce game" takes place at a private boys' idol-training school with a history of turning out many great talents in show business. The protagonist is the school's lone female student who transfers to the school because of a special situation, and enters the school's producer department.
Tsumugi, manager of the seven unique members of Idolish7, is pulled left and right by the energetic boys!
Green Leaves Entertainment is a tiny production company on the verge of going out of business in Sendai, the biggest city in Japan's northeastern Tohoku region. The agency once managed the careers of magicians, photo idols, fortune-tellers, and other entertainers, but its last remaining client finally quit. In danger of having zero talent (literally), the president Tange hatches an idea of producing an idol group. On the brash president's orders, the dissatisfied manager Matsuda heads out to scout raw talent. Matsuda makes a fateful encounter with a certain girl...