To be fair, as of the latest community update, it turns out he is right. In my initial response I argued with the information and assumptions I had at the time, but now Larian has given us official information which validates OP's claim. The hirelings are pre-generated characters and unless we can respec every aspect of them, our only way to have our custom parties is by the multiplayer method. Which is a workaround hailing back to the original BGs and not actually what was expected in a much more modern game.

Nobody's perfect... I'm a nobody.