So, the Fenris is a Frigate and not a cruiser.
The Leviathan would be like an ocean going Monitor, using the same hull.
The Aeolus becomes probably an AA Cruiser, although since it is smaller than a corvette this doesn't fit...
Corvettes, yes, those would be like Destroyers and Destroyers become carriers. On the other hand most Destroyers defeat most Corvettes (maybe except for some sort of Sobek vs Hecate matchup where the smaller ship outmaneuvers and orbits the other)...
Then if we compare destroyers, the Hatshepsut is more a battleship (a dreadnought at that, with a uniform all big beam primary battery), while the Hecate is more of a carrier with big teeth like the Kiev class.
The Orion would be a pre-dreadnought, with its mix of heavy direct fire beams and slashers.
It seems the only denominator by which ships get classified in FreeSpace is their size. All cruisers are small capital ships, all destroyers are pretty darn big. Corvettes are the stage in between, which appears when the GTVA's industrial capacity allows to build a large fleet of large cruisers (yes, this is also a historical designation, see the USS Alaska).
Now if we go to the real world, things are also very fuzzy. The Spruance Class Destroyer uses the same hull as the Ticonderoga Class Cruiser for instance, and the cruisers differ mainly by having the Aegis system.
The Arleigh Burke Class Destroyer does have the Aegis system, but it's a Destroyer nonetheless. It's also similar size to the Tico, being 8000+ tons while the cruiser is 9000+ tons.
Someone mentioned the Zumwalt - that Destroyer is nearly twice the size of a Tico, so we have precedence in reality that cruisers are smaller than destroyers... Whaddayaknow...