In RoP:tI there is the CrVi "Level 4: Create a temporary Arcane Connection to a demon within range." Apotropaic GL. My question is that can you use this GL in a similar manner to magically create an AC to a faerie or a magic spirit?
Based on my understanding there are similar rules for spirits independent from their Realm affinity. That is why my assumption is that if a GL is true for demons (which are essentially spirits without real body) it is true for other Airy Spirits also.
No, I don’t think that’s true. In RoP:I they do reiterate certain guidelines that are in the core book or elsewhere specifically for demons but if it’s not somewhere else listed mentioning “creatures of any one realm” or similar general statement then the guideline is only for demons. This is definitely the case for the guideline that replenishes infernal might pools, nowhere else is there a spell or guideline that seems to do similar for beings of other realms. Could someone say “this works on demons, can I find a way to do that on other creatures?” Sure, and then they need to do original research to create a similar guideline for another realm.
I would like to draw attention to a little (but IMHO very important) sentence in core (p 111).
This chapter contains a selection of example spells, and guidelines for what can be achieved with a formulaic or spontaneous spell of a given level.
Look at the word «selection». It does not claim to be a complete list of what can be achieved with Hermetic magic, even without a breakthrough. Except in a very few cases, I would merely allow extension of the guidelines by analogy, and the very few cases need a narrative. The most well-known such case is demons; their deception makes spells that work fine with other realms fail for them. Some cases will be wrought in mystery, the great Benjamin of Bonisagus tried for decades to devise the spell with no success, and nobody understands why. YSSV (not just may, should)
Demons are deceptive but I think they are also essentially reachable. If their purpose is to bring souls down to hell it makes sense that they are (or make themselves?) easier to find and summon.
Also the fact that to do some things with them is easier than to do them with creatures of other realms may not mean that these guidelines had only be discovered for demons and might be ready to research and apply to other realms, but that there is a plot out there to make magical users think that it is desirable to work with demons because it is easier and you can do more with less. Which sounds like something that must be written in the first chapter of Human Corruption 101 textbooks in hell's schools.
I know some folks here like to think that if something works in some place then it should work in all other places of that kind but I'm a big fan of asymmetry. I feel it to add context and color. It also reminds me that magic is not science, nor it should be methodical, extrapolable and congruent, because if it were then, well, it would be science.
I am a big fan of asymmetries with a story behind them.
Arbitrary and unexplained asymmetries are just confusing.
The point you make about demons being in some senses easier to reach is good; we just need to complete the story and it makes sense.
We tend to treat them as being all equal and stremlined, but there are things like:
Terram has 2 guidelines for it, the lower of which requires to be conscious of the attack.
Ignem has no "Ward" guideline, but uses "Control (element) in a slightly unnatural fashion" to do it.
There are some active spells, such as "The Wizard's Parry" or "Deflect the Wooden Shafts", which work at a range greater than Personal, not unlike an active defense, compared to the more passive wards.
Likewise, Control (element) in a slightly unnatural fashion guidelines, which are linked, occur at different levels accross different forms.
Just figure out that the Venatores, who had sworn to fight demons, discover that their Apotropaic Spells are actually a play from Hell, who willingly make things easier for them so they can interact with demons and that were they thought they were fighting the Infernal, they were actually taking advantage from it in order to stain hermetic magic with infernal magic!
Not to speak that also, whenever the Infernal shows an easier goal to accomplish something, some magi might turn into diabolists to take the shortcut. Houses had almost fallen from that before, and it might well happen again if the advantage is tempting enough.