Good questions! I've run into the yellow mushrooms a few times (Leucocoprinus birnbaumii) but was able to remove them and there root mass without hurting the girls. They only showed up at the edges of the pots so were easy to dig out.
Hey Lex, I love so many of your shows. The mushroom butts could be used to make mycelium and generate CO2 with a source (almost FREE,some supplies) that is already acclimated to the grow. Some Duckduckgoing will find some great examples.