Animae Magic (as seen in HoH:MC) is a really interesting ability with a bit of an ambiguous writeup. It's a cool, thematic ability that as a Major Hermetic Mystery should provide some real value, but it seems like it'd be very hard to adjudicate as Storyteller.
For both types: The Animae are apparently independent and must be bargained with, and come in a variety of pseudo-random personalities that would be more or less amenable to doing what you say. Of course there's the detailed method of taking into account all elements currently in play and making appropriate roll, but how about a shorthand for fast resolution, say a relevant skill roll with bonuses for favors offered against a known threshold like, 9, or 12? Or should it vary based on the Might of the summoned faerie (9+ Might/5 base seems reasonable)?
For Muto Animae effects, it turns and object into a faerie that is "usually something appropriate to its nature." There are no examples in the book, so how broad are the spells created with these guidelines? For instance, if I use the Base 10 Creo Animal Effect ("Change an animal into a faeries"), do I have to make a spell for each different type of animal (one spell for "Awake the Goat to Fae Ways" and another for "The Swine's Form of the Gentle Folk") or one spell that turns any animal into a faerie (within a given magnitude for size purposes)? Either option allows for varying amounts of prep-work to substitute for improvisation mid-adventure, if a player has a spell to turn glass into faeries you can work up some glass faerie power and stat templates you can apply, or if they have a spell to turn a spell into a faeries you can come up with some sort of rule for what a "spell faerie" looks like (probably like a theurgic 'spirit of a spell,' but with more independence?).
Creo Animae effects are more problematic. The text states that "Creo simply summons a faerie associated with the Form, and the maga has very little control over what." The specific form guidelines all have probabilistic and hedging words, "usually retain beastlike properties," "typically sanguine and flighty," "tend to be..." etc., and of them "Vim faeries are the most unpredictable of all."
To me it seems like there are a few ways to potentially handle this:
Total Storyteller Fiat: Coming up with a random, appropriately powered critter in the range on the spot every time is a bit of a hassle for the Storyteller, but cuts off some avenues of abuse in the area of the player picking specific, complimentary powers. Total Storyteller fiat is an opportunity to tune the received faerie to the included scenario exactly, but total fiat comes with the potential for players feeling railroaded or cheated when using their big fun power they built their character around gives them bad results, and on the other end you could end up making it an "I win" button with power sets or abilities that are too on the nose.
Systematize, Randomize: Treat Creo Animae spells as gacha rolls of faeries with random abilities suitable for their might (from page 92: "They often have powers appropriate to their Might..."). This would require creating a set of tables scouring the RoP: Faerie book and a decent amount of homebrew, perhaps assisted by a randomizer program, but after the initial inlay of work done at the outset it'd be relatively fast in play to make something random within the form. The Storyteller could, after generating the creature, tweak it, but the bones would be random. However, I'd worry about players repeat casting looking for "high rolls," fishing for the powers they wanted... or is that something to even worry about?
Tweak the Line: Actually, the maga has a decent amount of control over what they create. Allow (and by that I really mean require) that a spell using the guideline be specific about what type of faerie it summons, so that one spell corresponds to one creature guideline with several options in the description to optimize through. If they have a spell to summon a Terram dwarf, he's probably going to have a craft ability, but which one might be randomized, etc.. Requires storyteller / troupe work to come up with a template w/ random / unfixed elements for every spell of this sort.
Outright Ignore the Line: The maga has perfect control over what they create. One spell corresponds to one rules-following creature with no options, ready to party. Bespoke creations like this definitely allow for abuse via synergy / cherry picking the coolest faerie abilities or making your own, etc., but this is also the one that takes the least Story-Teller work, you just act as an editor and say "no, change this" until you get something back from the player you like.
For either of the last two there's the problem of players potentially casting spells with the guidelines you weren't ready for with spontaneous magic. This could either be disallowed (formulaic only!) or... I don't know what the or would be, really.
I think I'd lean towards the "Tweaked" version, it leaves a bit of randomness in the process without creating too much work for the Storyguides in the moment or a LOT of work up front (the gacha method).
I'm interested to hear what other people think on this topic. If someone's used Animae magic in one of your campaigns, how did you handle it? Are any of my ideas terrible or unworkable for reasons I haven't noticed? What sort of busted fairy ability would you be terrified to see in the hands of your players?
