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...
SOLDIER: Drop your weapons! Drop 'em now!
ANOTHER SOLDIER: 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.