I have been thinking about how Tal'darim could fit into the lore if Amon had not been retconned into the big bad. (Amend that: if Kerry never became the big bad either. If we need a psychotic bloodthirsty Zerg character, the cerebrate Gorn fits the bill.)
At first I thought the Tal'darim were an unsalvageable concept because their portrayal is inconsistent from game to game. Then I noticed something interesting: the Judicator caste "banned all xel'naga teachings", while the Tal'darim worship the xel'naga. Protoss religion is barely explained in the games despite being so vital, but since I find it interesting I decided to use this as the basis for my explanation.
Aiur's state religion has changed over the thousands of years of Protoss civilization. In the First Age (what I'm calling the age before the Aeon of Strife, based on the current age being the Second Age as stated in the manual), the xel'naga were worshiped as gods even if they acted merely as teachers and advisers. Eventually the Protoss turned against each other and the xel'naga, sending the xel'naga fleeing and beginning the Aeon of Strife. Not all Protoss turned against the xel'naga, however. When Khas founded the Khala as the new state religion, the Judicators banned all other religions including worship of the xel'naga.
It important to note that, technically, the word "Khala" refers to three different things: the system of teaching psionic discipline, the natural telepathic link, and the caste system. The Khala has built on the natural telepathic link to create the equivalent of an internet, using various technologies like psi-link towers to extend its range and crystal computers to store memories and speak with the "dead."
The Khala, in terms of comparison to Earth religions, is loosely similar to ancestor worship (though with lots of medieval Christian church overtones). When the Khalai talk of gods, what they are really referring to are their ancestors and not the xel'naga (this may be due to translations from Khalani to English obscuring the etymology). Using the Templar Archives computers salvaged from the First Age (as most of their technology is), they are able to read past life memories and even speak with the dead. Is is ambiguous whether the Khala actually preserves the soul of the deceased or the tech assembles a personality a la Zoe-A in Syfy's Caprica.
When social outcasts and rogue tribes refused to join the Khala, their reasons were varied as they were. The Sargas tribe feared losing their cultural uniqueness, the Tal'darim tribe(s) considered the Khala to be apostasy, etc.
While the Tal'darim tribe left with the Nerazim, they refused to circumcise and this made them strangers to the Nerazim. Tal'darim have their own telepathic network separate from that of the Khalai, but it lacks many of the features Khalai take for granted. The Tal'darim telepathic link is the same as their primitive ancestors, linking small groups whose members are in close proximity. They do not use psychic technology to extend its range or to store/access information remotely.
Another difference between the Tal'darim and the Nerazim is that the Tal'darim have continued to actively recruit from the Khalai. While Khalai are indoctrinated from childhood by the Khala, there are always a few social misfits and outcasts who the Tal'darim find and recruit. Certain recruits may be sent to the Nerazim if their personalities fit better there. Tal'darim may also recruit from the Nerazim, and have gone so far as to develop nerve grafts to reverse circumcisions. Ulrezaj is an example of a Nerazim who has converted to the Tal'darim, which explains why he employs them.
Furthermore, there have been multiple groups since the original schism that have called themselves Tal'darim. The word tal'darim is a pun that literally means "chosen" or "forged", which I am assuming is because those words are homophones. In myth it referred to servants of the xel'naga, so idiomatically it refers to angels, saints and apostles. Hence, it has been adopted by religious fanatics of many different stripes to promote themselves. I would not be surprised if the Tal'darim have a myth about their prophet being born of a virgin and returning from the dead, if the Nerazim are already drawing so heavily from Jewish and Romani symbolism while the Judicators are the Pharisees.
When the Zerg invaded the Protoss Empire (not just Aiur, they do have an interstellar empire), enterprising Tal'darim appeared out of obscurity to recruit in mass. This resulted in many converts and splinter groups appearing in the wake of the Great War, which the Khalai did not realize were actually a separate ethnic group and not just splinters of the Khala. These tal'darim used their punny name in the sense of "forged", denoting that they have been forged anew by the war with the Zerg. Likewise, Khala followers named themselves Shel'na Kryhas ("those who endure") to indicate their continued devotion.
The Tal'darim have studied their natural telepathic link and have developed applications that would disgust the Khalai. Not only have they devised nerve cord grafts, they have developed drugs (such as sundrop) that replicate the effects of circumcision without permanent surgery. Among the Tal'darim these drugs serve a variety of religious purposes.
What is not well known is that the Khala itself is addictive, and Khalai separated from it will experience psychosomatic withdrawal. Among recruitment efforts, sundrop is used to help Khalai converts overcome their addiction to the Khala.
Critique? Advice? Suggestions?
P.S. Note: Protoss words and phrases were devised ad hoc and not as part of a consistent language, but "na" and "nas" consistently meant "we" and "you" in multiple phrases. This may be the same "na" as in "shel'na." The "we" and "you" are personal pronouns, while the "those" and "who" are relative pronouns. Thus, "shel" may be a morpheme(s) which changes a personal pronoun to a relative one. The translation we got is idiomatic rather than literal. This would suggest that Khalani is not an analytical language like English.
P.P.S. I spent a fair amount of time trying to salvage the teleport retcon too, since it doesn't make sense that the pre-retcon Protoss would not use arbiters to teleport casualties off the battlefield. Furthermore, the SC1 manual disagrees as to whether dragoons were crippled, mortally wounded, or clinically dead when they were entombed. I decided to take the death explosion as a game mechanic and assume that in the fictional world a Protoss' "death" may vary in the degree of damage (leaving anything from a pristine "corpse" to a pile of ash) a la the inconsistent vampire deaths in the Lost Boys. I decided that Protoss have hardier brains than humans, so even one that has been "dead" for days may be revived as a dragoon (which also neatly explains why they don't just replace limbs or organs with bionics or something: the dragoons ARE full-body cyborgs).