But as I understand it, the game *must* have a frigate of every type (and cruisers are also frigates?), so also a siege frigate (cruiser).
Yes, sadly that is so, there is no difference between a frigate or a cruiser as far as the game is concerned. Even the corvette is just a frigate with an additional setting. That does not mean there can't be significant differences in model size and general toughness and weapon layout of the entity file. Current in-game Vorlon/Shadow "frigate" entity files and role type:
Shadow Carrier (Heavy cruiser/carrier)
Vorlon Destroyer (heavy cruiser) Shadow Stalker (medium cruiser)
Vorlon Siege destroyer (siege cruiser) Shadow Siege Stalker (siege cruiser)
Vorlon Drone (light corvette/scout) Shadow Assassin (corvette)
Vorlon Heavy drone (light frigate) Shadow Scout (destroyer/scout)
Vorlon Transport (utility frigate, various abilities)
Vorlon Envoy (diplomatic) Shadow Envoy (diplomatic)
Vorlon Life Giver (frigate colony, utility) Shadow Seeder (colony ship - seed is technically a frigate)
Vorlon Module constructor (builder of planet modules) Shadow Module Constructor (builder of planet modules)
Future - soon to be implemented frigates.
Vorlon Flagship (Victory condition frigate, unique role) Shadow Flagship (Victory condition frigate, unique role)
The Light Cruiser and Heavy Cruiser are the Vorlon Capital ships and by scale are substantially larger than the largest frigate. The Vorlon Titan is much larger still. TobiWahn has tried to the extent the game will allow to keep all models in relative scale to each other.
The Shadow have analogs except for the Shadow Carrier that is tougher and more dangerous than most younger races capital ships, but not as tough as the Elder races capital ships. It is very hard to balance the Elder race capital ships. The frigates are reasonably balanced except for the Shadow carrier, I'm still not sure if it is assigned an appropriate log points. The Vorlon Transport has no Shadow equivalent.