As well as being a newt nerd, I studied fungal growth a lot back in the day, don't ask why...
If a mushroom was placed in a terrarium, I think there would be a good chance the newt would eat it. Mushrooms give off pretty tantalizing scents that would be inviting towards salamanders...also, mushrooms in a moist environment will thrive and reproduce. As with any fungus, I mean, it also depends which you choose, they often go bad if the optimum requirements aren't met. I think it would be wise to let some grow and experiment in another small set up that is similar to what they would be housed in. You could see how the mycelium (roots) grow, and if spore reproduction takes place. Remember, some mushrooms are toxic, so research research research!!!