Several others have covered the fragility of game balance here (if you do want to go into something so complex, I would gather together a half dozen of the munchkiniest power-gamers you can and have them try to break the system before you even -think- about opening to the public), so I'll leave that alone except to say that gathering an alliance of PCs to overwhelm other opposition won't necessarily create RP... you'll get a group of OOC friends together who will stealth-group-app, having everything they need to support the wild-ass warships/troops.
No, what I want to address is the names used for the various ship types. They're utterly intuitive. Frigates and Corvettes work well enough, they're both patrol/escort vessels iRL, but if a Ship of the Line is a Battleship, it should be a heavy combatant, better than Cruisers or Battlecruisers (and certainly Destroyers). May I suggest instead the term "Monitor," "Assault Ship," or "Gunboat" (or if you want to keep the Age of Sail-y feel, "Galleon")? Then you can keep the "Cruiser" larger than it without dissonance from people who know ship classes. Next up, Destroyers are less powerful than Cruisers in modern parlance, and having it be the other way around provides cognitive dissonance. Perhaps using Destroyer for the ship between Frigate and Cruiser, and Battlecruiser and Dreadnaught for the two Space Noble ship types?
This may seem like a small thing, but when you have a complex system with a lot of the details hidden behind the curtain, you want the names to be as intuitive and clear as possible.
Totally separate, conscript units are generally useless for garrison duty on their home turf, as they'll just disappear as members desert. The term you probably want is probably "militia" or "levies."