A big part of the Alien Swarm mod I’m working on is spawning aliens so that they attack in massive waves, rather than in response to triggers set off by the player. Luckily for me, the game’s SDK already includes a lot of this functionality in its incomplete Holdout mode (operated by the “asw_holdout_mode” entity).
Packaged models should be viewable with the Model Viewer but Alien Swarm‘s aren’t. That’s because the Model Viewer requires a proper folder/directory structure even if said folders don’t actually contain any models. Otherwise you’ll get a bunch errors.