Yeah if the valium is legally prescribed to you, you should be fine in a drug test as long as you announce it and show them the prescription.
And opiates interact with benzos in a powerful way. You will definitely feel it, probably stronger than normal, but I don't know how much valium you normally take and your tolerance. Benzos and opiates are an easy OD combo if you aren't experienced in each drug and what you are doing.