@FlamelordYes, which is why I'm limiting their numbers this much. They're incredibly dangerous soldeirs, but there are only 300 of them. Considering the fact that a character such as Ultron is able to deploy armies of tens of thousands with relative ease, I believe it remains fairly balanced. It just means you need to keep in mind that you're not facing infantry. Treat every Space Marine as if they were a walking tank.
The reason I think the Alliance of Conquerors is fairly screwed is because they walked into a trap. The Space Marines currently have the advantage of cover, turret support fire and the collapsing buildings. Because the area near the palace is cleared the Alliance has no cover, and large parts of their army are getting crushed by rubble.
Of course, they might be able to turn the tide if they get good air support, or if they manage to break through the perimeter of space marines somehow, but considering that 200 or so space marines are firing in unison, it's nigh impossible to just charge through that. They'd have to deploy troops from the sky, port them in, or somehow prevent the Space Marines from firing in order to pull that off.
But maybe I'm being a bit too strategic about this ^^