The pure-blood heartless i guess always existed in the RoD, and Xeanhort just had the ability to summon them foe whatever reason. But the non-purebloods came into existence in the RoL through Xeanhort/Ansems experiments with people's hearts. Also I think it is safe to say that not many people in the realm of light knew about heartless. In BBSFM Aqua encountered the heartless and said something along the lines of "these aren't unversed" not "these are heartless"
But none the less, Heartless were not prevalent in the RoL at this point in time.
This is correct.
Pureblood heartless always existed in the Realm of Darkness.
In that flashback seen in BBS (which takes place 4 years before BBS no less) Xehanort "only" summons neoshadows, a pureblood variant, into the Keyblade Graveyard/Badlands which is a world of the Realm in-between.
So the notion "No heartless in the Realm of Light" in BBS is still valid as there are no heartless in the RoL.
Xehanort's power over darkness is so great that he can apparently summon neoshadows right out of the RoD with his own hands.
The emblem heartless however didn't exist until Ansem's and Apprentice Xehanort's experiments, and that was the reason for Xemnas saying to Ansem "you are the source of all heartless" => meant in relation to the RoL as it really were the experiments which resulted for one in the creation of emblems and the ability for pureblood heartless to enter the Realm of Light on their own volition.
---
@thread:
Regarding Unversed and Dream Eaters I would not be too fast labeling them as an one-time event just yet...