If by capitol ships you mean Star Destroyers, Mon Calamari cruisers and the like...they're mostly space furniture. The rebels get a SD to kill in Deep Space (us imps only get some poxy rebel space station), but apart from that the largest ships you get to kill are Corellian Corvettes. Aside from that quest that involves going inside a Corvette, you don't get to run around in any of them.
Corvettes show up over Kessel from time to time, accompanied by a pair of gunboats and a squadron of fighters. You have to take them out to get Ace at whichever pilot profession you take, usually.
Oh, and the SDs you see in orbit when you're on the ground? Mysteriously absent when you get up there. I still think the lesser TIEs should need to dock with them to hyper anywhere...