Short answers, pulled from the story.
A scriba in ancient Rome was a public notary or government clerk who worked out of the aerarium, the state treasury and government archive. They recorded sworn oaths, made official copies of government documents, and could assist magistrates in religious rituals by reading prescribed prayers aloud from official tablets. The scribae held the highest rank among the four occupational grades of the apparitores, the paid attendants of the magistrates.
Roman scribae received a good salary from the state treasury and could earn additional commissions for collecting and recording state revenues and producing official copies of government decrees. The Roman posting was considered so lucrative that the scribae worked in rotations, serving one year in Rome and two years in the provinces.
Gnaeus Flavius was a public scriba and the son of a freedman who won election as curule aedile in 305 BC, shocking the Roman upper classes. His victory was made possible by the growing number of freedmen and those of libertine descent in the urban population. The censors of 304 BC responded by adopting voter registration policies designed to curtail the political power of the lower orders.
Sextus Cloelius served as an agent of the populist tribune Clodius Pulcher and organized the ludi compitalicii in 58 BC, neighborhood new-year festivities that had been banned for promoting unrest. When Clodius was murdered, Cloelius led the people in riots and carried the body to the senate house, where it was used as a funeral pyre.
Yes, freedmen and their sons could become scribae, alongside educated men who advanced through patronage and even men of the equestrian order. The scriba's office was notably open to men of modest origins compared to most prestigious Roman positions.
The Augustan poet Horace identified himself in his first published book as the son of a freedman who held the position of scriba quaestorius, a clerk to the quaestors who were in charge of the public treasury. He used the specific Latin title to describe his civil service role.