Risa

Tertiary question: we know that Vulcans/Romulans were descendents of the Progenitors, but are Talaxians?

If not, it might not be technically possible for them to interbreed at all, meaning that a Vulcan/Talaxian hybrid could never occur naturally, and also that Tuvix would probably be genetically incompatible with everything and everyone. Therefore, no species - only an anomaly produced by a freak transporter accident.