That was right. Ok, basically a table file resides inside one of your .vp files, usually. Now, if you're going to change that table, then you have to create a new table file outside the .vp... darn this is going to be confusing for you. I think I'm going to go to sleep & try to explain this when I get up. In the slim chance that you'll be able to understand a brief explanation, here goes:
A table file contains settings for a particular set of objects in FS. (example, ships or weapons). ships.tbl would contain a ship's max speed, max AB speed, accelleration, vertical / horizontal slide speed (if any), etc, etc. These table files are normally stored inside a .vp file, and while they are there they cannot be changed. But, you can extract a copy of them to the correct directory (inside the vp files, there are directories.) The correct on in this case is ships.tbl. Now, these tables also have settings for if a player is allowed to fly the ship. So, if you extract a copy of ships.tbl to \FreeSpace2\data\ship.tbl & change the values, you can play as the Shivans (Well, at least fly their ships.)