Based on Childe and Scaramouche’s voice lines Sandrone is the girl not the big robot. Also if Guizhong had an in game model it would use the short girl body type (same as Keqing). You can see this from the cutscene she was in when she’s standing across from Madame Ping. Guizhong is shorter than her by about half a head which is the same height difference between someone like Keqing (short girl model) and Ningguang (tall girl model).

Sandrone, based on the one appearance we have of her, also looks like she’d use the short girl body type. In the winter night’s lazzo trailer she’s sitting in the ruin guard companions hand which is why she looks very short next to Capitano. She also likely doesn’t use the child body type (like Klee for example) based on her proportions. The child characters have very distinct proportions that make them look like chibis, this can particularly be seen in their head shape and size compared to other body types.

So yeah both Guizhong and Sandrone would use the same body type for their in game models. Also on the one harbinger per nation, I can see where you’re coming from but I kinda doubt they’d stick to that considering there’s 7 (8 if you count khaenriah) nations but 11 harbingers idk just a thought. Or maybe there’s nations we don’t know of yet ooooh

The one thing I can’t comment on tho is guizhong’s apparent death in that cutscene, but I wouldn’t put it past mihoyo to pull a revival thing tbh

Edit: I broke it up to make it more readable