For spellcasting drain purposes, are paracritters assumed to follow shamanism?

no. There's a sentence about it in Running Wild. I believe critters use WILL+highes mental attribute

Only thing I could find there was on page 213, under Innate Spell, where it states that they use charisma or intuition for drain at the GMs discretion.

FAQ'd, actually. An actually useful answer for a change:

Q: What attribute do critters use for Drain since they don’t possess a tradition?

A:Critters default to Intuition or Charisma when resisting Drain from power or spell use, generally whichever is higher; once an attribute it chosen it is suggested it be used consistently from that point on.