Accessing two types in a For...Next loop or function

Yes... but remember that if you've just got ellipsoid collisions, checking distances is all Blitz is doing anyway. I don't think you'd really be saving any time doing it yourself.

Code to make enemies fire would look something like this:

; in your main loop...
For E.Enemy = each Enemy
  if (Rnd(0,100) < 10) FireEnemyGun(E)
Next

; outside the main loop...
Function FireEnemyGun(E.Enemy)
  ; Make a new bullet
  B.Bullet = new Bullet
  B\Entity = CopyEntity(BulletSprite)

  ; Copy the firing enemy's position and rotation
  PositionEntity(B\Entity, EntityX(E\Entity), EntityY(E\Entity), EntityZ(E\Entity))
  RotateEntity(B\Entity, EntityPitch(E\Entity), EntityYaw(E\Entity), EntityRoll(E\Entity))

  ; Move bullet away from enemy so it doesn't blow itself up
  MoveEntity(B\Entity, 0, 0, 1.1)
End Function

The code to move all bullets and check them against enemies would look like this:

For B.Bullet = each Bullet
  MoveEntity(B, 0, 0, 0.1) ; Move bullet forwards on its Z axis
  For E.Enemy = each Enemy
    If (EntityDistance(B\Entity, E\Entity) < 1)
      ExplodeEnemy(E)
      FreeEntity(B\Entity)
      Delete(B)
    End If
  Next
Next