As mentioned previously, this assumes the third printing of the Swords and Wizardry rules, which I believe are still available in the form of the "white box" which you can download here. In the third printing, which was dated 2009, the only classes were Cleric, Fighter, and Magic-User, and the demi-human races functioned much differently and had fewer options. With the fourth printing, elves also may become thieves, and rather than the class being something inherent about how the race functions, the fourth edition assumes that each of the demi-human races have a set of classes that are available to them and thus they "multi-class" when they are using different abilities available to them. It also removes level limits on demi-humans in favor of multiclassing limits.
The playable classes of Secaelia are Cleric, Fighter, Magic-User, Thief, Dwarf, Elf, and Halfling. For an example on how to incorporate the Thief class into Swords and Wizardry rules, see the excellent example drawn up over at Akratic Wizardry or the "White Box" S&W rules (Refer to this page here for the Thief rules, plus much more). All subsequent material here will assume a "White Box" Thief, but the Akratic Thief is compatible as well, and indeed influenced the naming of the Thieves' Guild in this setting. Halflings can dual-class (just like Elves can alternate between Magic-User and Fighter) as Thieves; a character may also be a Gnome, in which case one still uses the Halfling template, however one may make stylistic changes for flavor. Here I will assume that, despite Halflings and Gnomes being different races, they are functionally very similar. Perhaps Gnomes may have a racial bonus on thievery over the innate abilities of Halflings, this is up to the GM.
Players should be made aware that playing as a Magic-User, Dwarf, Elf, or Halfling, or, to a lesser extent, a Thief, will present unique roleplaying opportunities due to the nature of the world of Secaelia. However, Clerics will be accepted nearly everywhere, and Fighting-Men if they seem reputable, and not like mercenaries. A party led by a Cleric, who engage in virtuous deeds, may have their reputation precede them, and open up unique and beneficial opportunities for them in more densely inhabited areas of the world. Dwarves and Elves may encounter discrimination based upon their race, while Halflings and Gnomes might originally only curry temporary disbelief. By and large, however, all Demi-Human classes are accepted within most Human society.
On the same token, Magic-Users are nearly universally held to be suspicious, and often being suspected of being a Wizard is a worse crime than being suspected of being a Thief. Nevertheless, using one's magical powers to help an individual or a village out of a bad situation might earn more renown than otherwise as the townspeople are impressed by a Wizard on the side of justice or righteousness (which many of them might have previously considered an impossibility). However, they will not overcome their prejudices rapidly, and a Wizard might have to work extra hard to distinguish him or herself as one of the "good guys."
Most Thieves belong to a secret society of thieves called the Sons of Akrasia. Membership includes knowledge of a secret language known as Cant, and an understanding of the secret symbolic code of Glyphics. Additionally, other members of the Sons of Akrasia will not rob them, nor their associates, nor those who have been designated by them to be off-limits (through verbal communication with Cant, or visual identification with Glyphics). Any Thief who is a member of the Sons of Akrasia can indicate any variety of things with Glyphics, and there are a wide number of symbols in the shared iconography; things like "keeps jewelry unprotected," "nosy neighbors," "loud pet," "leaves for extended periods of time," "corrupt constabulary," "judge," "experienced fighter," "strict penalties for thieving," etc. They are all symbols to communicate to other thieves what areas in what towns are good, or bad, for their trade. The symbols periodically change, so while it is possible for others to determine what the symbols mean, it is more difficult to keep up on the changes. The same goes for Cant, which is highly complex and constantly changing. Information is distributed through the thieves' network, and few are left in the dark for too long.
The last thing to note about thieves is that members of the Sons of Akrasia apppreciate the artistry of thievery, and do not consider themselves thugs. They attempt to avoid actual violence and harm, and conduct themselves according to their own special code of ethics. Most people who participate in robberies are not members of the Sons of Akrasia, and instead are just thugs or criminals. The Sons of Akrasia are, technically, criminals by their nature, but consider themselves to be a cut above the rest both in style and ability.
Halflings and Gnomes are adept at thievery, and are eligible for membership in the Sons of Akrasia, but nevertheless the organization still is overwhelmingly human. Dwarves are opposed to organized, sanctioned thieving by their nature, but nevertheless still do occasionally become desperate enough to participate in criminal activity. However, they are not eligible to become members of the Sons of Akrasia, and do not become Thieves. Elves may, with the Game Master's assent, take up thievery; in this case, players must refer to the fourth printing rules on multiclassing as opposed to the White Box rules. Thievery is something that comes naturally to elves, on account of their natural dexterity; however, other elves might disapprove of their choices, as thieving is not, to elves, an honorable profession.
A GM may optionally award the party points for notoriety and renown, in lieu of more conventional alignment, in order to keep track of how outsiders may perceive of them. In this case the points should not be intertwined; a party should be able to earn points of renown independent from notoriety, to represent a chaotic temperament over and above a lawful one, for instance. In other words, rather than characters choosing to be Lawful, Neutral, or Chaotic, their deeds may be judged as being Lawful, Neutral, or Chaotic, or Good, Neutral, or Evil. Regardless of the axis by which their deeds are judged, the GM can track whichever criteria they deem relevant and use these "notoriety" and "renown" points to color their social encounters as they establish themselves more as adventurers and heroes (or villains). Notoriety, for instance, might make it more difficult for them to hire Hirelings, while renown might make it easier. Renown might stimulate the local blacksmith to offer the group a special discount on his wares, while notoriety might make him shut up his shop when they are in the area. It is up to the GM as to whether characters will be able to "work off" points of notoriety, and also whether players will be aware of the actual count of notoriety or renown points at all.
The membership of a Thief in the party should not immediately cause an increase in notoriety, either, unless the party collectively agrees before the game begins that the Thief character is particularly audacious or noteworthy. However, a level one individual most likely will not have had ample opportunity to distinguish oneself in this way, so to already be notorious would be an exceptional quality. The same goes for parties which include a Magic-User character. That character's presence alone should not contribute to notoriety, unless they act in a way that is blatant, dismissive, and flaunting; in other words, unless they act in a way that is expected of wizards. Conversely, if a party contains any demi-human, they may get an "automatic" point of notoriety or renown, depending on which area of the world they are in, by the simple fact that they are travelling with that type of character.