I believe textures are the only reasonably reliable cross compatible element...and those seem to go roughly by "generations" (and sometimes figure gender): MillKids (PT Girl, etc.) take V2 texs, All the Gen3 (or Unimesh) figures take the same texs. Morph and poses may work, but not as exactly intended or envisioned because of various figure mesh and joint parameters...that's why the common qualifier of "with some tweaks". I guess it's that UVmap vs. mesh topology thing?! Yes, it would be great to have one of those compatibility charts that groups various figures by cross-functionality. I think you'd have to have experimented with every version of every figure to know how reliably character elements transfer. I guess such a graph would have various sections (Morphs, Poses, Textures)...interesting organizational challenge. Who's an Excel or PowerPoint wiz?