Why are there pixies, etc. on the First but not the Source?

[Iscah]

Also remember 12,000 years ago the Flood of Darkness tipped the balance of the remaining shards out of balance, and the first was tipped closest to light from the onset several millenia even before the flood.

12,000 years ago was the Sundering, by the Source's reckoning. The Thirteenth was flooded sometime after that, but time doesn't flow evenly across the shards so we can't be certain of that timeframe anywhere else - unless of course the various fluctuations of time roughly even out over millennia, which they well might.

In any case, several thousand years is plenty of time for tribes and races to spread differently on different shards with different histories.