If I remember correctly, Plutarch never says Cato actually fought as a common soldier. in fact I think the mentions that Cato was remarkable for only bring one servant with him. Also Plutarch says his great-grandfather lost 5 horses out from under him, so at that time at least Cato's family was of the equestrian class. He was certainly humble for a Roman aristocrat, but he does seem to have been at least a backcountry elite of some sort, having the renowned Manius Curius as patron. Poor and common relative to the great aristocratic families of Rome, but not a yeoman.