1. So, what is this type all about?
The idea is that Mushroom based species are specific enough to warrant their own type, as all Mushroom-based species use a type combination that was already in use or later became in use. The other reason is because mushrooms are not plants, though they are generally perceived as such.
2.Where does this name come from?
The name doesn't come from anything, per se. Spore's Japanese name is Mushroom Spore, but outside of that there is nothing specific regarding Mushrooms, however Mushrooms are their own archetype within the Grass-type.
3. Does this type have an in-game source?
Yes. Most spore moves and Grass-type's immunity to them is related to Mushrooms.
4. What would be the ideas behind such a type?
Mainly it would be based on different forms of fungus and mold. It would also be based on species with mushroom infection, and mushrooms' role in mythology.
5. What would be the type interactions of such a type?
It would be largely similar to Grass with less weaknesses and more effectiveness.
6. What would be the distribution of such a type?
Rare but easily available. It would be largely a mid-game type for uncommon encounters and status spam.
7. Future comments:
Mushroom based species generally appear in odd generations. If this becomes its own type, it would mean that more Spore users would be created per generation.