Stopping hitscanners from shooting each other...

This has probably been asked before, but, is there any way possible to prevent hitscanners from shooting allies, using any method (besides clever monster placement)?

You'll need to use DECORATE to modify the actor's behavior. One way to effect the change you wish, is to create a custom projectile for the actor's weapon, and make all actors of the same species immune to those projectiles. This will ensure that actors will not take damage from others of the same type.

Another way (which is probably very complicated) is to create "team behavior". In that way, an actor will not shoot (or continue to shoot) at a target when another actor of the same type gets in the way.