I don't think that's a niche that really needs filling. The Hecate packs enough of a punch to deal with anything smaller than a destroyer, and its firepower is diffuse enough that it can deal with several ships of that size at once. Where the GTVA is weak is dealing with other capital ships.
If you could build several frigates for the cost of one Hecate, sure, but that's a very difficult question to answer since our information on the engineering and economics of shipbuilding in FreeSpace is extremely limited. Off the top of my head we know that
- The Orion costs more to build that it costs to pay its crew of 10,000 for 3 years
- The Aelous was too expensive
- The Colossus took 20 years to build
- The NTF managed to build the Iceni in 18 months or less
If they could pump out Icenis they'd be very useful, they have as much firepower as a destroyer, are more manoeuvrable, are considerably smaller and therefore easier to defend and have 90% of the armour of the Terran destroyers, but who knows how replicable that design is. Maybe it uses loads of rare materials.