Granted. But, let's take modern-day frigates as an example. The british Type 23 carries a crew of 180. The german F125 Sachsen has a crew of 243. The american Oliver Hazard Perry, again, has 170 guys on board.
Given the sheer size of a Fenris, doubling that number just for purposes of damage control etc gives us a number between 300 and 500 guys, which does not sound that unrealistic.
Also, I am kinda assuming that a GTVA cruiser performs the same kind of policing/anti-piracy missions current frigates do, so you'd probably want a couple of warm bodies to do boarding actions and stuff like that.
Fair enough, but you're forgetting those vessels employ between one and two helicopters as well. So that's about 20 crewmen, not including pilots you've got on those figures.
I don't really see the logic in doubling the size of the crew just for sheer size. I mean, an aircraft carrier has a deck, several hangars, and lots of aircraft. Therefore it does make sense for them to have a large crew more or less. The largest part of a cruiser would probably be the enlisted mess, and it only has a few systems that need manning.
I've never, ever, seen a Fenris attempt to dock with another cruiser directly. I mean, there's the *small* problem of the ship getting torn to pieces by the 'dockee' vessel's defensive armament. That said, having a small complement of marines on-board to counter boarding actions in the event the vessel is stripped of its armament makes sense as previously stated. But even then, I'd suppose the GTVA might have a policy similar to the US Marine Corps, or the ADF, in which all members are trained in some basic IMT to provide local security, so in the event of a boarding action, those not on shift would comprise the armed crew complement.
The most valid argument I can see for a larger crew would be so you have 2-3 crew rotations. That would up the figure to about 200 people max, I think that makes sense.
Most destroyers are flagships. It's practically in their job description (it might be, actually).
Fair enough, Fleet CO usually resides on a destroyer. It makes very, very, little tactical sense to have a ship class dedicated for your Fleet Commander. So now if your destroyer were to bite it, you have no place within the fleet to coordinate operations, and the single largest weapon in the fleet is gone. It makes sense to have perhaps 2-3 destroyers, I mean, they aren't called destroyers for nothing! When there are concurrent offensives, or different parts of the system that fall under a fleet's jurisdiction, it makes sense to deploy more than one. Although, I'd imagine the Flagship had more C3 duties than the other destroyers.