Hi Sodales!
A new question about Muto Vim spells arise when I am creating a Hermetic Theurgist: can Muto Vim spells affect Magic Powers from Magic Creatures/Spirits?
My basic idea is: a spirit (from Hermetic Theurgist) cast a spell and another one (invoked with Diam duration) uses Passing the Reins of (Spell) to change the control of original's spell to the Hermetic Magus.
Thanks for the help!
The Muto Vim guidelines say:
"These spells can only be used on
Hermetic magic, as they depend on a good
understanding of the processes involved."
But spells cast by spell spirits (as when you use Hermetic Theurgy) probably do count as Hermetic magic rather than Magic Powers.
If your question is: Can a creature power work like the hermetic spell Passing the reins of (Spell) but specifically allowing the creature to transfer control of one of its other powers onto a second being.
Then the answer is firstly YSMV but to my mind the reasonable answer is yes.
Creature powers are generally designed like hermetic spells but they have much greater freedom to have powers that work in non-standard ways. Especially if you are designing the creature's powers as a Storyguide.
The real questions are: Why does the creature(or spirit) have this power? and how does this power advance the story(ies) that you are trying to tell?
I would assume that if you let players create a theurgist maga and then design their own spirits then this power is probably highly abuseable. I cannot off the top of my head come up with any specific abuses but being an experienced storyguide my abuse-spider sense is definitely tingling, and I would be extremely surprised if a savy player couldnt use this to pull off some shenanigans.
Spell spirits (TMRE p. 78ff) are essentially designed by players, as spells, because they essentially are the spirits of those spells. So that's just how things are.
As for Daimons... Those I'd probably want to craft myself, rather than leave them up to the player of the Theurgist. Except we play troupe style (or at least try to), so... maybe he's the SG next time?
Can spell spirits have powers beside the power to cast the one spell that they are the spirit of?
Ideally he would not use his time as storyguide to introduce Daimons for his character to summon and use 
No. Though there are two types, ones that can cast a single spell and ones that can cast any spell of a particular Technique and Form combo. It is also not a valid way to invent a spell that breaks Hermetic Limits unless you already know how to break that limit via some other virtue or it is no longer a limit due to OR in your saga.
To answer the OP’s question, Passing the Reins seems written to only allow passing “reins” from you to someone else so it would not allow you to take control of a spell cast by a spell spirit.
I guess I should have said the specific part of the OP’s question. There are other MuVi spells you absolutely can cast on an effect produced by another being or an item as long as said effect is within magic you understand which I assume means it would include hermetic-like effects from a spell spirit if you know Hermetic Theurgy.