idle thoughts.
To qualify as an mMF, there has to be specific spells, I know there is at least one for Familiars, but I can't recall where.
But does the mMF affect only plain Hermetic Familiars ? Or also Faerie Familiars? Infernal ones?
non-Hermetic ones?
Page 104 (bottom left paragraph) of the main book, it specifies that relevant foci apply, which implies that a MF in familiars should make sense.
If the magical focus applies would depend on it's wording: "magical animals" vs "dogs" vs "demons" vs "familiar"...
It's narrow enough it should qualify as a mMF, though in HoH:MC it's listed as a MMF. I say that because, even though it would apply to Familiars of any sort, including a variety of animals, a handful of spirits, Hermetic ones, Folk Witch ones, etc., it covers significantly less breadth than things like "birds of prey" or "canines." Of course, it is perfectly legal to select something ultra narrow for a MMF, even if it would qualify as a mMF.
As for specific spells, there are two canonical ones: Faerie Chains of the Familiar Slave (HoH:MC p.90) and Cutting the Cords (HoH:TL p.75).
I wouldn't quite say it's narrow given that, beyond spells, it covers the strength of the familiar bond and anything that can be enchanted in the cords, which opens up a vast repertoire of powers and effects, including constant non-warping enchantments. There's a reason it's Major.
Any focus that covers your familiar will also cover the familiar bond and anything that can be enchanted into the cords.
So a focus in "small, black, cats" would cover all that, if you had a small black cat as familiar, and I think we all agree that such a focus is narrow enough to be Minor.
See, I agree that a focus on cats would cover binding the cat as a familiar, and enchantments into the familiar bond that affect the cat, but I wouldn't let it cover effects enchanted through the cords that affect the magus, whereas I would with a magical focus into familiar because it's still a familiar cord enchantment. Hence why I think it's a fairly broad focus.
The rules explictly say that
Foci that cover the familiar apply to the
investment of all powers, no matter what they
do.
Yup. All arguments I've seen that it should be major have been based on missing the part of the core rules that ErikT quoted. Since all that is covered by a mMF in "birds of prey" or "canines," while both "birds of prey" and "canines" cover far more other stuff you can do than does "Familiars," "Familiars" should definitely fit within mMF.
Since the magical focus only applies to Familiars, how common are Familiars compared to now-Familiars?
How small a subset of an Art are they?
Larger or smaller than Healing?
Edit I left out the argument that Familiars are drawn from the subset of creatures that are willing to undergo the Familiar Bond once they have found a mage they can respect and trust.
At least, I am unaware if you can forcibly make a creature your Familiar.
I am not sure I understand your question. Are you asking about creatures that could potentially become familiars?
Obviously there is a much larger number of animals that could be suitable as a familiar than the ones that actually bond to some magus as a familiar.
But that seems fairly irrelevant to a focus in Familiars, since potential familiars aren't familiars.
I'm with ErikT here. I shouldn't apply to potential Familiars except when actually binding the being, as potential Familiars are simply not Familiars. Otherwise it would akin to saying my mMF in corpses applies to all living people and animals because they'll eventually become corpses.
Yeah, from the moment he quoted the rulebook on how a focus in the animal also applies to the master's enchantment through familiar bond, there ceased to be an argument for keeping it at major in my view (besides quoting the source of the focus, but I think the primary rules tend to trump examples).
It was my counter argument to the suggestion that a magical focus in Familiars should be Major
The funny thing with the source of the focus is that it still doesn't say it has to be major. Any canonical MMF that is narrow enough could be selected as a mMF. Too bad there isn't something below mMF for things like "wooden wands."
I think this one came out late enough that it was after the erratum so there was no longer a mechanical benefit of MMF over mMF for the same focus, though it was a pretty early book. So it would have been nice just to have printed it as a mMF.
Managed to check HoH:MC.
Was surprised. Always thought the canonical magical focus in Familiars was Minor.
Maybe the canonical MMF in familiars cover all familiars, and thus any spells trying to kill, buff, or debuff any familiar, also the one bound to your enemy. OK, it still only affects a small fraction of the animal population, but you can now do something you cannot do with a hippopotamus focus even if there never was any doubt that only a hippopotamus familiar would do for you.
Speaking from a pragmatic player perspective, you may start the saga committed to making the ultimate familiar, without having decided what kind of creature it is going to be. It could depend on what you are able to find during your voyages. MMF familiars would then be ideal. You don't want cats if you can find a dragon, and you certainly don't want dragons if they don't want to be your friend.
No, I have no idea if it should be minor or major, I just wanted to point out that there is no strictly-smaller-than relationship here.
But all those yummy components! 
Yes, certainly it applies like that. But still the number of beings you can affect with such things is far smaller than with "birds of prey" or "canines." Also, with the latter ones you can cast spells on people to turn them into birds of prey or canines, you can create birds of prey or canines, etc. These Animal examples give you both significantly more targets and more things you can do, so more breadth in multiple ways.
Why not?
For effects: How many Familiar effects that are not binding your Familiar can you make? Now how many effects that do not target an existing animal of a type can you make that use that animal's type as a focus?
For targets: How many Familiars are there in the world, even including Folk Witches? Compare that to just the number of types of things covered in the others, let alone the whole number available. For example, there are a few dozen types of birds of prey in and around Europe. Of each of those types, there are way more than there are Familiars.
Yes, that's a good reason to take a focus in "Familiars." Or maybe you want to have a few different types of Familiars over time (some Merinitae do this). But take it as a mMF, as taking it as a MMF really isn't ideal.
I would argue that it is not the number of beings that matters, but the probability that you will want to affect one. At least as far as game balance is concerned, probability is clearly the more relevant measure.
If this class be contained in that class, you would have a strict ordering. Here we just have overlapping sets of ill-defined entities which may be counted, weighed, or measured by size or probability in any number of ways. You choose count. Canon has not spoken.
Imagine you bind the dragon as a familiar first just to benefit from the focus when you harvest all those yummy components.