In her work on Roman public land, Roselaar notes that shepherds were often slaves , a position that one would not really associate with slaves for two main reasons: one, shepherds primarily act on their own. Two, in order to be effective at keeping away wolves and such, shepherds need weapons, something that the Romans were usually very eager to keep out of the hands of slaves.
In other situations, historians such as Livy describe populist leaders recruiting slaves into their causes. Although Book 111 does not survive, we do have a summary of it. In that summary, the epitomizer includes the following on the year 48 BCE “When the praetor Marcus Caelius Rufus stirred up revolt in the city by inciting the plebs with the expectation that debt would be cancelled, his magistracy was terminated, and he was driven from the city and joined forces with the exiled Milo, who had assembled an army of fugitive slaves. Both men were killed while they were preparing for war.” (trans. Chaplin). Sometimes when historians make claims like this, like when Sallust accuses Cataline of recruiting slaves into his army, it’s hard to take such accusations seriously (if you read the beginning of Sallust’s Bellum Catalinae it’s very clear that he really hates Cataline and is perfectly willing to slander him). Therefore, it’s entirely possible that Milo hadn’t raised an army of fugitive slaves. But then again, maybe he did?