Hey.
I don´´t think that (in general) it is a question of proper orchestration if you understand the simgers or not. It´s more baout the thing, that some (if not to say many) singers are really weak in clear articulation.
I did a lot of underscore work in the past and I didn´t thought too much about which istrument shold play which range. If you have to score a dialogue scene you see (and hear) what is happening and so you choose what music to write.
I´m not a big fan of rules like: "Never use this and this instrument unisono under a dialogue". It always depends to what you are working on.
Best regards,
Stephan