I'm assuming this is for Ren'Py and you want to use the NVL mode for internal dialogue and ADV for the actual spoken out loud one, right? I can't see why not!
Yep for Ren'Py! Basically the main story would use ADV but I am thinking of showing letters via NVL mode so I don't have to have the main character read out each line. I was worried it might count as non-gui asset for this reason. Is that still alright?