The game directly contradicts that information.
[Ultima Brand Fragment:
Words of Protectors Past
The Heart of Chaos does not grant the Guardian power-it eats away at their physical self. Caius did not gain an immortal body, but was cursed with eternal life.
Heed my words, Noel. You are the last protector, and the only one who can wrest the Heart's strength from Caius. In the instant you strike the life from his body you shall either inherit the curse, or beak its eternal grip. Only your will can sway fate one way or the other.]
-----
[Chaos Brand Fragment:
Words of Protectors Past
A massive battle once raged on Gran Pulse. Paddra was besieged by a great army that sought the life of the seeress. In the face of this assault, Caius chose to perform an 'incarnate summoning.' This ritual merged his essence with that of the Eidolon Bahamut, and drew forth all of their latent power.
An incarnate summoning expends the life force of the summoner, and thus did Caius perish. But he returned from death's domain, an immortal heart beating in his breast. That dark summons is now his to wield at will-as long as the Heart of Chaos pumps eternity through his veins.]
------
[Gorgyra Fragment:
Best of the Beastiary
There was once a female warrior of Paddra, bound to serve the goddess, who was tricked by an enemy and almost caused the seeress to be killed. Now, the seeress loved the warrior like a sister, and she forgave the mistake. But the warrior did not, and as penance she asked the fal'Cie to turn her into a Cie'th.
When the warrior finished her prayer to the fal'Cie, an ominous voice rang through her head. 'I have heard your plea. You shall lose your five sense, and be plunged into black flames, and wander the eternal darkness. Your words shall be a song of agony; your screams a curse of death. And thus shall you remain, until a merciful one destroys your soul and frees you from your torment.']
-----
The Datalog entries for myth and history record the information that Noel and Serah find out through the story; Everything in them is supposed to be something one of the characters heard from another character. They're meant as a guideline to Noel and Serah's understanding, and the Fragments go above and beyond that. There were Guardians before Caius, such as Gorgyra, and they were l'Cie with the power to wield some form of Chaos. Caius alone has had the Heart of Chaos, so there actually is NOT a 'long line of eternal servants'.
What Serah and Noel think isn't necessarily right. For example, at the ending when Caius is dying Serah says that he must have always wanted to come to Valhalla, to die for real so he and Yeul could be together, and rest for eternity. So that when Yeul died, she wouldn't have to come back to him. This is absolutely not the case, as Caius' goal was to merge the two worlds so there would be no future and no visions to kill Yeul anymore. 'In The Goddess is Dead', Caius mocks them and how they followed his plan without ever knowing the truth. The point of the story's perspective is, again, to give the player more information than the characters have. The characters are tricked, and rather than the game simply following their knowledge and leaving players in the lurch you're afforded the opportunity to find the truth out for yourself.
It's a design choice. As I said, it's not how I would prefer games to be made. But I understand the direction they're coming from, and what they were trying to accomplish.
As an aside, I recall Toriyama stating in an interview that Lightning 'did not fight with full force' once she realized Caius had Etro's heart inside of him. The only thing we know to be true is that Caius killed his own predecessor, and we don't know that he HAD to. Since the developers went out of their way to mention Serah passing on her Focus and claiming it to be similar to how Guardians all have the Focus to protect the Seer, I'm willing to put stock that they don't have to be killed. Caius being the exception, because the Heart has to be stilled.
EDIT: Oh, and here's a Fragment regarding the fal'Cie relation to people who can travel the timelines. Or, more accurately, are blessed by Chaos like the Guardians are.
[Book of Avalon Fragment:
Words of the Seeress
Hark, and hear the secret words of the Farseers.
Only those blessed with chaos by the goddess may travel the portals of time. Even the fal'Cie, with all their great power, could not enchant the blessed and make them turn against the goddess. The goddess;s protection is nearly absolute.]
-----
If this is true, then any Guardian would be immune to the natural course of the Focus given them by the fal'Cie and be free to perform the duties tasked them by the Farseers and their goddess. This is why I believe Gorgyra had to pray to the fal'Cie; It appears she had to give up Guardianship to become a Cie'th, unless Etro's protection doesn't extend to volunteering.
As for the story being a mess, that's your opinion and you are of course welcome to it. However, I understand most of the story and what I don't understand rings of questions left unanswered for Lightning Returns. I found the plot enjoyable despite it being a little on the nose.
At any rate, I think I'm gonna take a break from this particular discussion. (The Guardian stuff) I've been having it over and over again across different sites the last few weeks, and I am just burned out on rehashing all the same points and counter points with people. xD Thanks for the debate, though.
love these details, really.
Good work, though -- fuck.
I'm Leaferian (Leafé for short), and my friend (whose username is Mwynn on here apparently) told me that I should come by here and get involved in the discussion! 
.
Gilfinder II, Rolling in CP, Item Collector II, hello. I'm gonna do the Archylte Steppe monsters now for a lovely 120,000 CP and some cash, and then see how strong I can make my monsters for the final Paradox Ending battles...
