I'd just stick to solitary or circle for org.
Didn't you know that every season (or in response to major emergencies), the gorris of a forest gather together to discuss matters of import to the all the shroomlings of the forest?
They mostly sit around in a ring smoking, chewing mushrooms and gossiping, but occasionally something useful arises from their meetings:
Organization: Solitary, ring (2-12), or circle (1 plus 5-20 shroomling)
Advancement: by character class. In fact, I think I'd change the urdin shroomling advancement to that, too.
Advancment by character class is good for me as well.
I like the Urdin getting bigger with age, though, so how about:
Urdin Advancement: 1-3 HD (Tiny) or by character class
Gorri Advancement: by character class
CR of 3 maybe. They do have a bit more combat capability, but most of their extra abilities are for support.
We need to include the Fighting Fungus in our CR estimate, remember. A standard gorri could create up to three of those using its Spores SQ.
How much would one of those be worth do you reckon? I'd guesstimate a weak CR 2 - they do have 4 attacks, Reach, and a bunch of immunities from their Plant type.