The "average" urban inhabitant was an impoverished day laborer who live on the edge of starvation and was semi-homeless. In all likelihood he would die in the same condition he lived, probably relatively early due to the disease rampant in the more crowded parts of the city. He had no education, although he might have some functional literacy. He had next to no connections. If he was lucky enough to know a trade he might be decently well-off, but only a minority of the urban poor were skilled laborers of any kind. He owned no property of his own and he and his family (which might be quite large, given mortality in the city) lived essentially a hand-to-mouth existence daily--in short, it kind of sucked.

The older view of the urban poor as a parasitic "idler population" living on state largess has been thoroughly torn to shreds by this point. The urban plebs had to work to survive, and work quite hard. Day-laborers in the city either found work or did not--a day without work might be a day without food. State-subsidized grain, initially with a reduced price following the Gracchi and then for free following Clodius, maintained limits on the amount that could be collected per family, and though it might have been barely enough to feed an individual it wasn't close to sufficient for a family. The more regulated grain prices of the Augustan Period helped the urban poor to purchase additional grain on the market, but in the late Republic the price of grain was notorious for fluctuation, both by natural famine and artificial manipulation and market speculation. Further, free grain was only available during a brief period between Clodius' tribunate and Pompey's assumption of the cura annonae--under Augustus a further requirement, a hereditary token, was added and the state-subsidized grain became the hereditary privilege of a relative few. Besides grain (most day laborers could probably afford little else to eat on a regular basis) and the need to buy clothes, rents in the city were notoriously high. They were often paid daily, which might have been somewhat accommodating given the need to migrate through the city looking for work, but their high cost probably meant that for many laborers a day without work might have been a day without food or shelter. To pay for all this were wages, paid daily according to the ability to find work, at staggeringly low values. Cicero's comment of a denarius a day for even an unskilled slave is almost certainly inflated, but even if we take it for the norm it's depressingly low given what we know about the cost of food and rent--for a family, which might have multiple members unable to work but consuming resources, it might be devastating.

If this new opinion of the urban poor is rehabilitating in comparison to the "idler population" of the past, it also has serious repercussions when speaking of the involvement of the urban poor in politics and their social mobility. There's something of a debate about how "democratic" the Republic was, and central to it is how much the urban population, the vast majority of which was horribly poor, actually was involved and kept up. There's a great deal of evidence that suggests that no matter how much the urban poor might have wanted to be involved (a question which itself is debatable) a good chunk of it simply could not. A day listening to a contio might be a day's work missed, and with it an empty stomach. Those who missed work, we can be sure, might have found themselves able to participate--there are lots of propositions regarding what the work-less did on their off days. But ideally this didn't happen, at least not often or by choice. Besides this the urban poor was a rather impermanent population. Disease swept through the slums of the city, and insulae caught fire or collapsed constantly due to shoddy building techniques, more concerned with saving construction money than good workmanship. Seasonal laborers entered the city during the slow part of the agricultural year to make a quick buck or two at the monumental building projects or down at the Tiber warehouses--with them they brought new diseases and caught some that they had never been exposed to before. Even the "real" urban population was semi-migratory, since their constant search for work and daily rents probably forced many of them to move around periodically. The case has been plausibly made that those most capable of regular political participation were the so-called plebs media, those members of the urban poor who had skilled trades and possibly owned a little shop or business of their own. They were hardly well-off, but they were not so wretchedly poor as to be unable to leave their shop for a day for fear of starvation. We hear of fabulously wealthy freedmen like Trimalchio, mostly people who inherited some wealth from their former masters. Most freedmen were part of this plebs media, however--in fact, they may have dominated it. Our epigraphical evidence is problematic, but something like 60% of goldsmiths, to take a common example of city workmen present in inscriptions, are freedmen--of the other 40% nearly all are slaves. There are problems with assuming this is strictly representative, but it is certainly telling--the number of skilled freeborn laborers in our inscriptions is incredibly small, even given the general trend for freedmen to be more prevalent in epigraphic material than poor freeborns.

"Social mobility," no matter how we're defining it, was probably out of reach for most of the urban poor, barring a colonization project or the like. Contrary to what you suggest, military enlistment was rare in the city, most soldiers were country boys. For an unskilled laborer the chances of getting out of abject poverty were slim--with a skilled trade this was more likely. It's probably not that surprising that freedmen appear more common in skilled trades, since many freed slaves would have been trained in a trade while slaves, and had close ties with patrons that they could take advantage of to help start businesses--for most freeborn day-laborers, many of whom were coming into the city for the first time, such connections did not exist. Skilled workers, and therefore freedmen, were more likely to rise to minor or local magistracies--a Hadrianic inscription set up by nearly 300 magistri vici (the minor elected magistrates who oversaw the vici, or neighborhoods--their duties and power in the late Republic are uncertain, but under Augustus they were more or less just there to maintain local cults and little else) lists only 13% of its members as clearly freeborn. In the Augustan Period membership in the vigiles or other minor associations might be a good way to start moving up, although this was probably a dead end. More promising might be work as one of the attendants of a magistrate, the apparitores. The most famous of these were the lictors--this office was often held by ex-centurions, but apparitores included scribes and messengers and so forth, which were frequently freedmen but could also be of course freeborn, usually members of the plebs media. A patron's influence was often crucial--one famous inscription set up for a deceased freedman notes that this freedman's son was able to secure a military tribunate thanks to the help of their patron and his former master. Patrons generally had much closer ties to their former slaves, however, than to just any poor shit, and of the poor shits that weren't their slaves familiar faces like the butcher and other skilled laborers were more likely to receive help--who the hell remembers the poor day-laborer, a face in a sea of faces, who might not even be alive a week from now? From there there might be ways forward, although it might take years or generations--your children were more likely to get somewhere than you were