Do it! If it is still docile at 13in, it may likely remain. I know you know the potential risk so you'll need to watch. But if I were you, I'd do it. I've kept a docile mbu with rays before with no problem and I have seen it done several times.
Should have just bought yours back off me. LOL It's been good with my rays for the last 2 years. I currently just switched it to my pup tank as the pikes were harassing it too much.
Been two years with a mbu 13"+ in my pup tank and never any concern on either end. The mbu will usually sleep on a ray or my frt depending which one is in the corner it likes.