I remember on one mod (cannot remember if it was Phoenix RPG or Monkey U) where I made the turrets for cap ships to only be used by what they where supposed to be on. Such as, you could not put a Battleship turret on a cruiser or gunboat, which they had many problems with people doing just that.
Now comes a question
Could we do the same for engines by giving them classes for different types of ships? Say a light fighter has a class 1" engine heavy fighter has a "class 2" engine and so on all the way up to battleships.
Something like hp_gun_type except it would be hp_engine_type = hp_engine_special_1
Now this is just a thought I had. I have no idea if it will work or not but thought I would put it out for discussion.