To differentiate *what* sizes needed? If you mean player count, Duty Roulette: Mentor can put you in 4-man, 8-man or 24-man duties of differing role requirements and with everyone being a forced Freelancer in Frontline, all you need is 72 eligible players for Frontline, which is only 50% more than Rival Wings.
The system in this aspect is in fact advanced enough. If you mean another reason / size, let me know.
That said, Rival Wings is in dire need of promotion beyond silly moogle tomestones and a separate Daily Challenge or one sharing with Frontline will definitely benefit it, short and long term.