No, not all the Alliance forces were destroyed.
Some find the Crew after Mal sent out the message. The Operative orders them to stand down.
From the script:
Behind it, grappling hooks chained to a huge tractor pull the
wall away as through the smoke come some fifteen Alliance
soldiers, who line up, rifles trained on our gang...
Drop your weapons! Drop 'em now!
Do we engage? Do we engage?
We don't know for sure exactly what happened to the Reavers.
The Firefly Wiki makes an unsourced claim that the entire Reaver Fleet was destroyed:
They engaged the Union of Allied Planets Navy in the Universe battle and were annihilated.
Knowing that the Alliance ultimately won the battle, combined with the Reavers' blood-rage likely preventing them from retreating even in the face of near-certain destruction, and the fact that taking Reavers prisoner is almost impossible, I would agree and say that they were probably all destroyed.