I would never take mushrooms for the first time after having come down from MDMA. Yes there is a sort of afterglow from MDMA but there is also just basically a crash. Any such physical and/or mental dip is to the disadvantage of your mushroom trip.
In other words yes the chance seems significantly larger that it turns sour. I do not like to talk in terms of 'bad trips' because I believe that difficult experiences tend to be opportunities to learn from them. But having said that, other experiences are just completely shitty and side-effect riddled. In those cases the term bad trip probably applies, I guess. They happen less often than I think the term bad trip is applied though.
It's smart that you don't hippyflip (combine) for your first time either, that is generally recommended against since it is wise to first get to know a substance well before you explore variations on it. That includes variations in set & setting by the way. The first time should be a default situation safe at home with every risk minimized. Trip for the first time well rested and clear of mind, emotionally stable.
Have a really nice time rolling tonight, trust me and forget about the mushrooms and remember that there will be a next time for the mushrooms. Also be prepared for the possibility that while rolling you might think you feel so good that on the comedown you might eat the mushrooms. Avoid such a situation, you should make sure you don't change your mind because you are incredibly high.
You think you can find yourself in that answer?