Also keep in mind that this is an era where there are essentially three sets of nobility - the Napoleonic, the traditional titled nobility (duc, marquis, etc - these were generally earned through force of arms), and the more recent ancien regime nobility (in Toulouse, these were generally connected with the courts and the Parlement and called the "nobility of the robe" - they were NOT looked on well by the old families, the "nobility of the sword"). The nobility of the robe are not the only type, but they're the major type of this, rather akin to the English baronet - one is granted nobility for services (or upon paying a large sum, or accompanying a government appointment) but without a title, merely the granting of the particle and the rank of noble (the particle being equivalent to "sir" and is inherited). Moreover, various others were permitted to append a name via a particle to their family name, usually the name of an estate. These are generally equivalent to the English gentry of the time: old families holding land for generations, where different branches are granted the opportunity to separate/differentiate themselves. Example: the Martin family has done damned well for themselves and has basically divided into two branches based on the major lands owned, one branch near the village of Lépaud and his cousin near the village of Gouzon. Upon petition to the king, M. Martin from Lépaud may be granted the right to call himself "M. Martin de Lépaud" as his whole surname. After a generation or two, the originally family name was frequently dropped, so you then had the related families of Martin (still in Gouzon) and de Lépaud. You'll see this movement described by Balzac in Eugénie Grandet - one of her suitors is going through this process.
This is entirely separate from the later movement where people with perfectly boring names like Dupont (literally "of the bridge", generally given to someone who lived near a bridge or was a bridge keeper) would space it out as "du Pont" merely in order to sound like they came from one of these great families. Degas' family did this - his father and brother were known as de Gas, but Edgar quit going along with their bullshit as soon as he was old enough to start signing things for himself. The actual famous du Pont family in the US is actually "Dupont de Nemours" - Mr Bridges from Nemours.
Courfeyrac is probably the son of a noble of the robe or equivalent - an ancestor bought himself in or was a judge in one of the major courts or something of that nature (could have been a royal appointment some generations back, but I think the two possibilities aforementioned more likely) - and is being very egalitarian. I suspect he would find a "de Gas" hilarious.
What kind of literature and what kind of life is the same question. - Tom Stoppard