yes its fine as long as you're used to subs... it might dull the trip a tiny bit, but not very much at all... as for morphine, don't take too mch or it will ruin your roll... so a small amount of opiates with MDMA is not a problem. just don't take too much!!! if its for going out raving clubbing, i would keep the opies for after, if its a chill at home with some friends a little bit is fine, just DON'T mix subs and morphine....
and its not about how long after the molly you can take the bupe, you can judge that by feeling (probably 6-8 hours after dropping the MD)
safety wise, no problems