Since the gemstone summons literally just pop in for a move and then out, I honestly don't see why they need to be an 'entity' that replaces the Carbuncle. If they were suddenly converted into animations it could have absolutely the same practical result.

Demis are more complex since they linger around for 15s, but at the very least it would be more or less a minute of the carbuncle actually being able to be involved in something else.