Maybe, but that depends on how well the youkai can be organized. Some of them, like Yuuka and Remilia, might be too proud/aloof/wrapped up in themselves to follow orders or group together under someone else's banner. Yukari might help, but good luck catching her awake. The Moriya shrine is a surefire ally, as Kanako gets faith from youkai and doesn't want her source of faith drying up. On a related note, the tengu are already organized and would be likely to pitch in on Byakuren's side if it became apparent Miko was a threat to them. Ditto for the kappa.
The PC lineup for a game involving the war would likely feature Reimu, Marisa, Sanae, and either Sakuya (as the servant of a Devil), Aya/Hatate, or Momiji. Nitori is a possible candidate too.
