Proposed Canon
Neutrality as a concept is rather rare in Erfworld, with only two known entities actively practicing it: the Magic Kingdom and Charlescomm. While most sides engage in constant alliance building and warring with their neighbors, both Charlescomm and the Magic Kingdom lay claim to neutrality with regards to wars that rage across the Erf, allowing units from each to work as mercenaries on a contract basis for any side that is willing to take them. The unique political and unit structures, and overwhelming magical power they have at their disposal, of each have allowed them to survive outside of the traditional alliance and expansion systems of conquest.
 The Magic Kingdom
from the main Magic Kingdom page
The Magic Kingdom is not a side, and does not have city sites which can be built up through moneymancy. It is the domain of barbarian casters of all types, who can interact with their counterparts from all around Erfworld in a community removed from the constant war and politics of the many warring sides. Casters from enemy sides rub shoulders with Barbarian casters, many of whom are permanent residents of the Magic Kingdom.
All the permanent residents of the Magic Kingdom are Barbarians. They were either in the Kingdom or otherwise safe from disbanding when their capital fell,Erf-b1-p102 or they popped as barbarians. Given that some Warlords pop spontaneously, it's likely that a much smaller number of Casters do the same.
These resident barbarians survive by working for various sides by contract (as mercenaries, or selling spells as a service), selling scrolls for Shmuckers, or live in the kingdom selling services for Rands and trading those for rations and upkeep.
from the main Charlescomm page
Charlescomm sells the services of its units as mercenaries, even those of its Ruler (only those available remotely). It deals in anything of value, and does not operate on cash-only policies. Although Charlie (the primary dealmaker of Charlescomm, as well as its Ruler) will consider any offer, his reputation as a reliable mercenary is important to him, so Charlescomm will normally not change Sides in a conflict.
Services known to be offered by Charlie himself include contract Thinkamancy and manipulation of employers. The Archons, Charlescomm's primary off-site consulting Unit, can provide significant combat casting abilities based on Shockmancy and/or Thinkamancy, and Foolamancy as well as a range of other services such as spell security. Clients choose which services they wish the Archons to provide. Archons interpret contracts themselves, and have on occasion provided more than the client contracted, but this must not be relied upon since it is up to each Archon to decide whether to go beyond the scope of the contract. Charlie will also hire casters from the Magic Kingdom as contractors for specific jobs.
The main feature that unites both the Magic Kingdom and Charlescomm is their heavy reliance on caster-type units. Since casters and archons qualify as warlords, and have a certain level of free will and ability to make their own decisions, they make excellent mercenaries. However, their motivations are vastly different.
The Magic Kingdom has fashioned itself as a haven for wayward casters, a community of learning and sharing that, due to the immense power at its disposal, cannot be seen to be interfering in the outside world for fear of reprisal. The balance of power amongst the casters seems to be the result of mutual self interest (or mutually assured destruction). If any one side were to gain control of the Magic Kingdom and its inhabitants, then all sides would lose the benefits that it offers. Further, that side would become immensely powerful, having access not only to the Portal network, but all of the knowledge and resources in the Kingdom, making them a threat to the entire world. Thus, neutrality is a survival strategy for a sub-class of powerful units that, through the benefit of free-will, can maintain a level of independence outside of the status quo.
Charlescomm, on the other hand, functions more like a traditional mercenary force and business venture. Charlie really seems only interested in maintaining his business and ensuring that there continues to be enough conflict that he has a steady stream of customers. Whereas the Magic Kingdom uses its neutrality as a survival strategy, Charlie's neutrality is a branding mechanism. Most sides that work with him know that he could easily turn on them and fight for their enemies, if he so chose. Or, he could simply send a huge wave of archons to decimate their kingdoms. However, Charlie comports himself as a neutral party that is only interested in making money and is, therefore, trustworthy up to the letter of his contracts.