Since the slaves were armed and mobilized and the owners of those slaves took a deal the government offered (apparently 2 slaves for each one freed), did Rome honour the deal, and if so, would the slaves become citizens? Or were they just sent back to the owners since their services were never actually needed?
The slaves remained under arms for some time. Ti. Gracchus, the grandfather of the famous Brothers Gracchi, was Master of Horse in 216 and consul in 215; he took the slave army into Italy where he had some success. Gracchus used the slave status of his soldiers and the offer of liberty as a goad. For instance, at Beneventum Gracchus first offered freedom (Liv. 24):
Whoever brought back the head of an enemy would be at once by his orders declared to be a free man; whoever quitted his place in the ranks he would punish with a slave's death.
This proved counterproductive:
The military tribunes reported to their general that not a man of the enemy was being wounded as he stood, whilst those who had fallen were being butchered and the soldiers were carrying human heads in their right hands instead of swords. Gracchus made them at once give the order to throw down the heads and attack the enemy, and to tell them that their courage was sufficiently clear and conspicuous, and that there would be no question about liberty for brave men.
But he went back on this later:
At last Gracchus gave out that there would be no hope whatever of liberty if the enemy were not routed and put to flight that day.
Gracchus won the battle and freed his slave soldiers, and they remained serving with him.
Gracchus died at Lucania in 212. The army seems to have judged their loyalty more toward the general that awarded them their freedom rather than to the state (Liv. 25):
The volunteer slaves who had done excellent service when he was alive to lead them, looked upon his death as discharging them from further military duties and accordingly disbanded themselves.
This and the destruction of Fulvius' army caused a bit of a panic at Rome. The consuls rounded up the survivors of the two armies as best they could, including the volunteer slaves, and they participated in the siege of Capua.
A relative of Gracchus, Blaesus, brought Fulvius to trial for his mismanagement of his army, and Livy makes him give a speech in which he compares Gracchus, who made an army of slaves into citizens, to Fulvius, who had an army of citizens and reduced them in demeanor to slaves.
Later they show up in the legions of Livius, and that's the last we hear of them.