well, if analogy to real life Capital Ships is called for this is the first place I would go.
http://en.wikipedia.org/wiki/Carrier_battle_group
from that page, typical composition of a U.S. carrier strike group:
1x Super Carrier (typically Enterprise or Nimitz class) and its Carrier Air Wing
1x Destroyer Squadron (generally 4-6 ships, for example http://www.cds7.navy.mil/default.aspx)
1-2x Aegis Cruisers (Ticonderoga class, equipped with lots of cruise missiles and RADAR systems)
2-3x Guided Missile Destroyers (Arleigh Burke class, generally lots of surface-air-missiles and anti-submarine weapons)
up to 2 attack submarines (usually Los Angeles class)
1 supply and logistics corps (actually lots of tankers and transport ships trailing behind the main fleet)
thats 1 capital ship, 2 large offensive support cruisers, and 6-9 smaller defensive ships (the destroyers), as well as optional stuff like the submarines.
so you can see that in real life naval strategy Cap ships are never used alone and are not considered self-sufficient vessels. they drive the strategic value of the fleet because Cap ships have capabilities that simply cannot be replicated by smaller ships but they are not competent to defend themselves alone in surface fleet action and require a large escort at all times.
world war 2 era battle groups were of similar composition but multiplied by a factor of 4 or more.
basically the cap ships in Sins correspond pretty closely to real world cap ships. not at all self sufficient but clearly the most powerful and most important vessels in their fleets. they provide unique and powerful strategic capability but they aren't all-in-one invincible super death machines.