Forerunner frigates are sentinels. While the forerunner capital ships are individually overwhelming, the frigates display technical prowess by being mass produced.
Evidence can be seen that Onyx's surface was made of trillions of sentinels. And also 10 weaker ships will deal much better against the flood than 1 massively powerful one.
Yep. Even without a certain ability all Sentinels have, there is literally no better race to fight the Flood than the Forerunners. The Sentinel spam can wipe out the Flood just due to sheer numbers - we don't care if they capture 50 of the ships b/c there are 500 of them in the fleet.