Mixing generic prescription cough syrup and OTC syrup?

Before anyone tries to comment on my lifestyle, or what I'm doing, don't bother. You aren't going to change my mind unless you provide credible information that shows it's dangerous.

My question is: I'm doing syrup this weekend with my boys. I've done it several times with just OTC Robotussin, as well as just generic codeine brand prescription medicine (lean, sizzurp, etc.). However, I only have about 1 ounce of the presciption stuff left, and want to mix it with some Robo (it's only active ingredient is DXM, with dosing @ 15mg per 20 ml.). Is it safe to make this combo? I usually run about 5 ounces of Robo, so I plan on using about 3 ounces instead to avoid overdose because of the potency of the codeine. I weight 140 pounds.

Thank you for your help!