Aside from weed, as in combination with most things it does produce absolutely profoundly differential effects, at least so much that it woudl be considered "mixing" with it (that may just be me, as smoking pot is just something I do).
I would separate the DXM and Ambien, from the LSD and mushrooms side of the equation, IMO they don't belong together.
As someone said earlier DXM and probably ambien would destroy the "pureness" of an LSD or mushroom trip ( although this is very subjective), in both the form of the effects, and the mindset of the experience.
Aside from all these opinions, I doubt there is any substantial physical health risks to the combo (other than doing something ridiculous)