I was thinking...Can Metasapients use the Shapechange spell? I could see some very good uses for it: The Naga with it who uses it to disguise him/herself as a metahuman and live among them. The Pixie who uses it to get past corp-sec, or even to go out and go shopping. But, that would only be if it was possible to use...

I don't see why not - most metasapients are closer to animals that normal metahumans.
Hitch is, unless they clarified it anywhere else, a lot of GMs would probably argue that "normal (non-paranormal) critter" means "animal". Even if it allowed metahumans, anything except "human" might be considered "paranormal".

So, unless your Naga thinks it would have better luck fitting in as a snake, or your pixie would rather shop as an eagle, I'm not sure what the benefit would be.