I'm very leery about adding ships. I downloaded a large passenger/freighter thingy once and the game played fine but the ship didn't. It wouldn't hold station in a convoy. Convoy zigged and the added ship just went straight.
I shot the convoy with the bow tubes and than turned the stern towards the stray and set up a vector analysis shot and missed, I NEVER miss this type of shot. What I found was the ship not only wouldn't hold station but speed as well. It just keep slowing till it would eventually stop.
A nice fat target but I had to take it back out.
Magic