Monsters using "Missile" attacks can cause injury to any monster in DOOM EXCEPT members of their own species. That is to say, an Imps' fireball can hurt any monster in DOOM except another Imp. The same holds true for any other species of montser that fires a missile.
There are two special cases regarding this:
Occasionally, a PWAD designer wishes to create a Doom level based on an actual place (such as his office building or home). I undertook such a project using the actual blue prints as my reference. This creates the problem of equating Doom measurement units to real world units. There is no magic formula. The Doom engine does not appear to render things with consistent aspect ratio. For my project, I found that the following scale produced a satisfying result that "felt right" when walking through the level:
16 Horizontal Doom units = 1 horizontal foot
10 Vertical Doom units = 1 vertical foot