I would agree on the seven "current" Princesses of Heart, as the main reason why most hearts in the "current" universe even have darkness is because the true KH, which is all light, resides now in the Realm of Darkness. When a new heart is born from KH, it has to travel the RoD in order to reach the RoL and be "born" completely in its designated body. During these travels the newborn heart in question is "tainted" by their exposure to the RoD. I would assume that the longer the journey for the newborn heart to its body is, the more darkness it initially has due to longer exposure.
This would mean that i.e. Sora's and Ventus' hearts traveled rather fast while Maleficent's or Pete's hearts traveled slower.
Going by this the current princesses are actually a paradox, lol. It's strange that these can retain their absolute pureness without the faintest glimmer of darkness.
Except Ventus, who's pure heart is created artifically we also don't know about any other HoPL out there.
I would probably even go so far to say that the "seven princesses" didn't even exist before the keyblade war as then everyone had a HoPL, it wasn't something special then.