Starsector 0.97a is out! (02/02/24); New blog post: Simulator Enhancements (03/13/24)
you do that with a system hardpoint, I think
You can't have more than one going, no. If you specify more than one, it'll just use the first one. If you don't specify any, it should use the center of the ship.
In short, if you throw a stone out of the rear window of your spaceship you will go faster.