I've said this somewhere else, but I think that "frigate" is used in FS more to designate a special-purpose vessel than to identify a specific warship class. "Medical frigate" is actually a class that goes back to Star Wars, and it just sounds right, while the Iceni is more of a heavy corvette used for the special purpose of Bosch's command ship/ETAK platform. And although the Iceni is bigger than a Deimos, it doesn't really stand up that well in a firefight, so to call it a higher order ship in terms of firepower is sort of unreasonable. My version of the FS designation system, by size and firepower:
1) Escape Pod - Not going here. Lets crew escape, otherwise a big missile if necessity forces.
2) Drone - Unmanned, small craft, potentially armed but only lightly. Contains minimal systems necessary for space flight and navigation. Drones are not currently employed by the GTVA, and only make cameo appearances in training simulators.
3) Fighter - Small, single-man attack craft, capable of mounting a wide array of basic pulse armaments and missiles.
4) Bomber - Single- or two-man attack craft, more bulky and heavily armored than a fighter but slower, capable of mounting anti-cap ship bombs as well as pulse weapons and missiles.
5) Transport - Unarmed or lightly armed military or civilian transport craft, capable of transporting large numbers of crew or limited raw materials in sealed, pressurized internal bays
6) Freighter - Unarmed or lightly armed military or civilian transport craft, with no internal storage but dockpoints for a variety of specialized cargo containers.
7) Cruiser - Warships up to 600 meters in length, lightly armed and armored, generally used to fend off fighter and bomber attacks, and operate with minimal crew (100+).
8) Corvette - Warships between 600 and 1,500 meters in length with no or partial fighterbay, 15-30 turrets, and a medium crew compliment (750+)
9) Destroyer - Warships between 1,500 and 2,250 meters in length, with full fighterbay, 30+ turrets, and a large crew compliment (5,000+)
10) Carrier* - Destroyer sized ships with less firepower and more fighterbay capacity.
11) Superdestroyer - Warships between 2,250 and 5,000 meters in length, with a 40+ turrets or a superweapon. Extensive armor and very large crew compliment. (12,000+)
12) Dreadnaut**/Juggernaut*** - Warships over 5,000 meters in length, with 50+ turrets (up to 90 or more), a large fighterbay, extreme armor, and a massive crew compliment (20,000+).
*Not used by :V:
**Not used by :V:, can be used alternately for Juggernaut
***Only assigned to what was initially believed to be a Shivan superweapon; should not be used to designate a warship of the GTVA.
But hey, this is just me.
And if you are making a TC of any kind, sticking with the FS designations isn't really helpful anyway.