It's possible that they literally can make ships faster than we can kill them. There's no way we can disprove it.
So guess it's time to find whoever else lives in the Milky Way, and make as many allies as possible.
Returning to the topic:
There's room for both a fleet of smaller ships (usefull for patrols and surprise attacks from safe directions) and a few larger ones in the battlegroup (these would be fire support in larger battles).
It won't be as numerous as a task force made fully of small units, but thanks to the capships there will be room for many fighters (the only field where the GTVA fights Shivans more less as equals untill overrun) and some serious firepower.
From a tactical point of view:
The fleet should not be concentrated around anything, except the ships in action (ie. the corvette that got ambushed should automatically get everyone's available escorts untill it can jump out to safety).
There also must be many supply and repair ships among the fleet, to keep as many of the ships in action as possible.