Much as I love Sephy, I think he's being overestimated here.
Sephiroth is not equal to entire armies, he can be severely impeded by a backstab from a random MP. He'll need to sleep at some point. It would be a bad move for Shinra to go after him, but when have Shinra ever done the sensible thing? They could just grind him down like Zack, if they were willing to lose enough soldiers and machines, and could pin him to one location. Pre Jenova, he doesn't have the phasing through matter/flying/teleportation abilities, which would make it easier. Genesis, his nearest equivalent, had an army of super loyal deserted SOLDIER at his back and still couldn't really do much damage to Shinra, even with Sephy mostly not intervening.
Re the Tsviets, Azul and Shelke are toast, Nero wouldn't be a match in physical ability, but his swirly black stuff seems to be nearly unbeatable, and you don't stop it even by killing him. Rosso could survive Cloud, so might be able to put some fight.
Huge emphasis is put on how crazy powerful Weiss is, he's kept chained up. I would still give a match up to Sephy, but I think it could be a close fight.
Imagine all the super fans swarming to rally around him, ShinRa would be over in a day
If all the One Direction fans decided to unite against the world governments, they'd cause a serious problems, but I'd still give the military the edge in a fight.