While I would prefer a yuan-ti be added over this, I think that if the 'template' is added, I think +2 ECL is just a bit too much and I think it should instead be +1 at most, and want to get the discussion going before too much effort in implementing them is invested.
Comparing the stats of the Tainted Ones to the six other supported subraces/template we can see some trends.
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As MAB mentioned, you forgot to include Quick to Master in your Tainted One entry. Also, out of curiosity, why showcase only 6 of the many ECL or non-ECL subraces? What were the criteria? And finally, it's a template, not a subrace, though, like blooded one, it's restricted to one racial type, so it's a pseudo-subrace I guess.
Is there a sort of metric you use to evaluate the quality of the racial features to ensure that it is in-line with the power of comparable other subraces?
We adhere, for the most part, to the PnP level adjustments for ECL calculation. If you're interested, they go into detail in the second chapter of Savage Species on how a level adjustment is determined.