Is there anyway to make it so there are a bunch of people that spawn in at the start of the level, but once they get killed they can't spawn in again? I want to do this for realisem.

Make the units on the field very high, set reinforcements to near-zero in the lua.

Manually trigger an ambush at the start of the map.

But if this is a hex-edit question we can't help you there. It's a lot more difficult to build around a pre-compiled file.