First, the rules for creating Fairy seem to start at creating Companion level characters and above. But being able to just Summon Companion level characters at will seems awfully powerful. Particularly as the powers they can have don't seem to be limited by Might. I.e. a Might 10 Fairy can cast Lvl 50 spells. It then seems much easier to summon a Might 10 Fairy that can cast a Level 50 spell than to research a Lvl 50 spell.
Example: CrVi20 (base 4, +1 Touch, +1 Diameter,+10 Might) to Create a Fairy that then has Greater Power: PeVi50 Demons Eternal Oblivion might cost 5. And then can take Improved Powers to reduce the might cost to 0. I know the Penetration would be awful in this case, but thats not the point, its inventing a Lvl 20 spell to cast a Lvl 50 spell.
Second, when use you Intricacy Points to reduce the Might Cost of a power, does that affect Penetration. In the above example, he would have Penetration: 10(Might)-5*5=-15 And so could not actually affect a Demon, but if the cost is reduced to 0, then it utterly destroys Might 10 demons (or higher if he spent Pretense on Penetration).
Third, under Creo Vim guidelines for Animae Magic it says they are designed as Daimons from Mysteries(revised). These have unique powers like Lasa Vegoia the Matron that has Teach Theurgy to teach any Hermetic Theurgy spell (which it does instantly and then dissappears, and the student then needs to think over for a season). The player is tring to redo that as Teach Animae Magic, but still, it has no limit on it. So a 10 Might Vim Fairy could teach a Lvl 50 spell back to the mage. And then he could do a CrVi50(base 4,+1 Touch, +1 Diameter, +40 Might) and so on.
For scale, the game has only been going on for a little while, the players have had 3 years out of apprenticeship for their magi. The player that is playing this requested very early that he wanted Animae Magic to be a central idea for his character, and a quick look at the bestiary doesn't seem to make them too bad. but its only once it came up that we are starting to see the problems.
Vim faeries are treated as daimons? Whoever wrote that probably failed to distinguish between an actual daimon and a magic spirit; a simple spell like that should not be able to create a faerie with unlimited Aspects.
Anyway, I would not give a manufactured faerie like that access to any really unique powers, and especially not teaching abilities (that, if I'm not mistaken, breaks a Faerie limit).
With anything that allows summoning, whether Hermetic Theurgy from The Mysteries, Animae Magic for the Merinita, Sihr for Sahirs - all summoned creatures should be firmly subject to storyguide veto. Yes, it's a lot less work if you let the players create wonderful things to summon but if you can't trust them not to create super-duper overpowered assistants of doom, then you shouldn't let them.
Low might faeries being allowed to have level 50 powers: This is entirely possible in the rules as written, but such powers will have subzero penetration, so anyone with any Magic Resistance can ignore these powers, so a level 5 Infernal imp or an annoyed faerie of very low might can resist these. Of course, any magus with parma will also resist these so really it just allows someone to use a faerie to blast mundanes with big spells, so it's not as big a problem as you might think. If it annoys you too much, insist that the faeries can only have huge powers to affect themselves or illusions, all powers used on others must be of a level roughly equal to the faerie's might.
Vim animae being like Daiamons: the only Daimons listed in The Mysteries who teach are both Might 30, so you can say that from the examples shown, you don't feel comfortable letting anything with a might below 30 have teaching powers. If you really want to be restrictive, say that all created spirits that teach can only teach what their creators know - the examples from the book are one who teaches mystic theurgy and all its spells (if she came into being from a mystic theurgy cult, it's entirely reasonable to assume her creators knew a lot of theurgy) and one Roman Daimon of military honor who teaches leadership to level 6 (which is about the level you'd expect a roman general to have).