Then again, only the original Defiant and the Valiant of the above lot would have been built to be Borg-fighters. The others would in all probability be later keels, built for a wholly different job. And perhaps to very different specs, too - they might lack ablative armor (as its presence on the Defiant came as a surprise to Leyton's people in "Paradise Lost") and probably would lack cloaks, and perhaps quantum torpedoes as well (since no torpedo action took place in "Message in a Bottle").
Six of the ships might have sufficed against the Borg if they had worked as planned. Sixty might be a good counter to the Dominion as matters turned out, and not even six hundred might make a difference against the Borg in final analysis.
Timo Saloniemi
Well, the fact that the Defiant had ablative armor at all was unknown on the whole to Starfleet Operations. Given that the Defiant was the pathfinder vessel of its class why wouldn't later vessels in the same class have that advancement? It was never prooven to be wasteful.
The Cloaking Device I'll grant you, as the Romulan government only offered one to Starfleet to use on the Defiant and then only within the Gamma quadrant for purposes of gaining intel on the Dominion. But, then nothing in that says Starfleet had to construct the Cloaking device itself. Quite the contrary.