There are 4 capital ships: Hutt and Pyke in orbit of Tatooine, Crimson Dawn in Toshara and Ashiga south of the Kijimi Nebula. The trick is to turn off collision damage in the options. You ram the ships (not the back, otherwise you lose health) until they are weakened and then you finish them off by shooting. And of course you also have to have a good reputation with the factions so they don't attack you on sight.