The men with silver hair are almost definitely Sephiroth clones. Also, the guy on the wheelchair is definitely Rufus. If you watch the new trailer on the official AC site, you can see that the wheelchair man has a white suit and looks young. I'm not sure what Rufus wants with the white-haired men or why he had the Turks and Cloud fight them, but the white-haired men are surely looking for Jenova, assuming that they're clones of Sephiroth. There were many parts in the new trailer where they talk about their "mother". In the motorcycle chase scene, Loz asks Cloud, "Where is mother?".