I would have a hard time describing it as "safe." Mostly because every drug has its risks and its more useful to consider them even when minor or unlikely.
I wouldn't think that 15mgs of XR addy and an extra strength monster would be seriously life threatening to someone who is of average health. I do think it will strain your heart, which could be potentially uncomfortable. Making the combination a habit may increase the damage to your heart caused by long term stimulant use. And as Jaurk mentioned, it may cause anxiety. However, I'm unaware of any particular ingredient in Monster drinks that would make the combination severely dangerous.
Remember, everybody's body is different. Make every decision cautiously and weigh the risks/harm with the benefits/pleasure. Odds are, it wont kill you or cause noticeable long term harm. Still, its not a particularly healthy combination.