It would seem you should be able to avoid setting off an explodestool in a similar manner to getting through an entangled area (move at half speed).
I'd prefer it if they normally exploded automatically, but you could pick your way through them with a Reflex or Balance check.
I'd also set the weight limit to 8 pounds, the standard minimum for Small sizes.
Oh, and I was bundling the explosion damage of all the explodestools in a patch together to get the 1d8 damage. If you have 1d6 mushrooms in a patch doesn't that mean they can do up to 6d8 damage? That seems too much.
e.g.:
Explodestools (CR X)
Explodestools appear to be small mushrooms of an edible nature, which grow mixed with normal mushrooms of similar appearance. A DC X Spot check or DC X Knowledge (nature) check is sufficient to detect the presence of explodestools within a patch of mushrooms.
If a creature weighing over 8 pounds steps in a 5 ft square containing explodestools all the fungi in the square explode, dealing 1d8 sonic damage to and deafening any creature within 10 ft for one hour. A DC Y Fortitude save negates the deafening. An explodestool will similarly explode if it is plucked or takes any weapon, force, or sonic damage, including sonic damage from the explosion of other explodestools. On the other hand, any fire, cold, or acid damage destroy explodestools without setting them off.
If a character recognizes the presence of explodestools within a patch of fungi, he or she can avoid setting off the explodestools by moving at half speed through the patch and succeeding at a DC Z Reflex save or Balance check.