I'm of two minds about it - obviously it's something that the game directly supports. On the other hand, constructing the ship is a large chunk of the actual gameplay.
Oh, I guess that's what "-Constructing the vehicle outside the bounds of the VAB or Hangar" means. I assumed it was referring to mods that give you more space. But I guess that's redundant with the "no mods" rule.
In the stock game, you are able to position the ship so that parts of it stick out the roof of the VAB. One of my super big Eve missions had to do this. Any reason why that wouldn't be legal?
I haven't contacted either of the other mods about this, but my current opinion is that premade ships are fine. As far as I'm aware, premade ships are only available in Sandbox mode, and time spent constructing ships isn't counted in Sandbox mode anyways. Plus, I'm doubtful that any premade ship is the optimal ship for any category.
If I'm wrong and premade ships are available in Science and Career mode, then my opinion is to disallow them in Science and Career but to allow them in Sandbox.
