Thanks for clearing that up. So let me see if I got this right. The seven princesses keep the realm of light from being overcome by darkness. And when the princesses are in trouble the x-blade shards will choose the guardians that will protect them.
That's the basic gist of it. Or at least how Xehanort and Yen Sid imply it to be. Although the "how are the guardians chosen" is a bit iffy since Xehanort implies that they are chosen naturally (or by fate, as he likes to talk) while Mickey and Yen Sid apparently just work to gather at least seven people who can swing a Keyblade.
If the latter is a working method though I would wonder why they would stop just at seven and play into Xehanort's hands instead of recruiting more than seven and eradicate Xehanort's thirteen weirdos instead of risking the forging of the X-blade.
Now what I would like to know is what can that 1/7th power do. What are they capable of, Kingdom heart is the the source of everything and all, and the total focus of evryone in this game yet we still don't know what it can do?
So theoretically are the preincess of heart demi gods of some kind? I know they are not equal but still
Well, what we DO know is that
1) The Princesses can sense and feel darkness (in KH 1 FM the Princesses at Hollow Bastion sensed Xemnas' immense power and informed Sora of it before the Unknown fight took place, also in KH 1 Kairi subtly sensed that something was wrong with Riku, which is why she made that weird speech to Sora about how Riku has changed. Sora didn't get what she was implying though).
2) They can block and hold back darkness. (In KH 1 after the final Keyhole was opened a flood of Darkness came forth which the Princesses blocked off until Sora had found another way to return to Hollow Bastion (with the new Gummi from Cid) to seal it).
3) They apparently can restore disappeared existences, like when Kairi brought Sora back and gave him a friggin' new human form.
Especially point 3 is of interest since it comes together with the notion of KH itself being the source of life. While the individual Princess may not have the power to
create life, they may have the power to restore a disappeared being to physical form as long as there still remains something of that being, namely the heart.
I think I do not need to elaborate too much how this would play out in regard to Eraqus, Naminé, Xion and Roxas. If Sora can awake and free their sleeping and trapped hearts (as he's destined to and can accomplish with the help of Ansem's data), the Princesses may be able to help so that they can actually
physcially exist again.
In short, Sora restores them as their own, independent entities in the spiritual sense while the Princesses give them the container they need to actually exist
physically.