The current scheme of fleet-building works okay for conventional warfare (in FS terms), but since the Sathanas tactics of the shivans are definitely not on with the Terran-Vasudan standards for waging war, and it would kill our available resources to build our own Sathanas-like fleet, I suggest deviating from the norms. Rebuild fleets half with standard vessels and half with a new class of vessel: The Strike Cruiser/Corvette/Destroyer. Normal size but with fewer guns and less armor in favor of more speed, maneuverability and heavy weapons. Imagine replacing the front of a Deimos with a beam cannon. The ships would not be made for prolonged engagement and would rely on fighter cover for defense, but they would be capable of striking from long ranges and for massive damage. 2-3 corvette-sized ships could take down a Ravana or a Demon, 4 could declaw a Sathanas. In terms of stopping Shivan invasions like this, they would prove an invaluable, cost-effective asset.
They get in, hit their target, and get out, giving the "standard" vessels their window of opportunity.