Sure, it's possible, but I'd assume that most of the time they'd follow you instead. Furthermore, if I choose to head to the hilltop, they're pretty much certain to follow. In any case, I need somewhere to put the other half of the Marines - killing them will provoke the anger of the others, letting them stay will stop the Banshees from spawning (if I remember correctly), and if I put all the Marines together, they'll shoot the Banshees to pieces. This would actually be a nearly ideal set-up for me: not too hard to make (I have to use Heroic, not Legendary), gives me options, and ensures that Marines still play a part. The only problem is that if I head to the end of the stream, the Banshees may not follow entirely; but if that happens, I can just return to the checkpoint and try again. At least, this is what I would assume.

Vale,
Perseus