I'm not saying they can't connect, I'm saying you're making them connect mid-game. Elves they've seen before - sometimes, somewhere, they're practically everywhere. They've not seen Gazaryas though, because I just made those up. Now, when you encounter one in the game it's a big deal, because the player doesn't know it, he might know what to expect if you told him, but he'll never entirley know. It's a feeling you're invoking, and it's not something to be taken lightly. If you're world is built around that, like Planescape, great - But consider that Planescape is supposed to be weird, you're supposed to now know anything and you're supposed to feel lost. This is not something you want to invoke in all worlds all the time. Using the established fantasy races gives you the power to make the "ordinary extra-ordinaty", that wonderful feeling of a fantasy world you feel at hom in. Now, a world that's entirley known is boring, but a world that is entirley unknown is confusing, or scary. The gold is how you mix them.