For every 256 destroyed ships you will get the message: "Right On Commander". This is an indication of your advances between the ranks. From the table you can see that you should get Dangerous between the third and the fourth message of this kind. You advance to Deadly between the 11th and 12th message. You should become 'Elite' after the 23rd message.
The type of ship you destroy makes no difference to your Combat Rating. Whether you kill a Panther Clipper that has a Plasma Accelerator, or a Gecko with a Pulse Laser, it still counts as one kill and 256 points.