The Summon Suthra Ritual in the Magic and Martial Arts guide references a Suthra's Power Pool. Is that the standard number of dice a Suthra has in the Beastiary? Otherwise, how do you determine the Power Pool for a Suthra?
Hello! And YES! You nailed it exactly! For most circumstances, including their reaction to being summoned or manipulated with magic ritual, the Power Pool of a creature is its Normal Action Dice for most of the regular things it does. There may be instances where you might want to use the Special Actions dice associated with the creature, but only if you feel one of its "Special Actions" would apply to its reaction to magic for some reason.
The rules in the Magic and Martial Arts book are essentially expanding upon the essentials provided in the Basic Compendium. On page 283 of the Basic Compendium, it states that ..."Summoned Suthra have Power Pools (the amount of dice used when attempting any action with them) and other statistics, equal to the amounts indicated in their descriptions within the Bestiary of this book."... What this indicates is that to Power Pool of a creature is essentially the pool of dice it rolls based on both the Normal Action Dice and any Special Actions they may have, as indicated in their Bestiary entry.
The important thing to remember when it comes to Suthra Summoning (as opposed to demons, spirits, or elementals), is that generally the GM decides which suthra from the natural world show up to fulfill the summoner's desires. The summoner may not even know what's coming, just that nature will hear his call and hopefully provide what the summoner needs (based on their successful rolls, of course). That means the GM needs to pick a creature whose either Normal or Special Action Dice represent enough skill that they have a good chance of actually succeeding at whatever it is the summoner needs. In the Magic and Martial Arts book this becomes evident very quickly as you begin to read through the various Disciplines Suthra Summoners can achieve.
For example, in the Bearer of Burdens Discipline, it states: "The Power Pool of the creature or swarm, therefore, should be within the ranges of Action Dice identified in the Bestiary of the Basic Compendium for the chosen creature." And based on earlier indication of the kinds of creatures that would actually heed a summons in the first place, it states: "Unless the sir’hibas is interacting with a particular breed of suthra in some way, the species of the summoned creature depends on the current location and desired task. This Ritual focuses on the needs of the summoner rather than a specific species of suthra. Such needs may include attack or defense, having a steed or beast of burden, sustenance and resources, or the desire for a messenger or scout." ...and also, in the "Way of Prithya" (the initial Discipline used to reach out to a creature to summon it) it indicates that: "When using this Discipline, the character reaches out with his mind and feels the primitive sensations of suthra in his immediate vicinity, within a radius of several miles. This enables the sir’hibas to find a suthra who might be able to fulfill his current need... It's all about the summoner's need, and so a really good roll of the chosen Discipline should produce a creature that has a really good chance of fulfilling that need. And a wise GM would not only figure out what that creature would need to be, but would also insure (even if it means adding extra dice to its Normal or Special Action Dice) that the creature has enough dice to have a really good chance at success.
Hopefully this helps!