Can't have a familiar with a soul

(Spirit) Summoning, Spirit Familiar, and Theurgic Spirit Familiar all allow you to bind spirits, which includes demons.

As a Tytalus,
I had spirit familiar, and bind my master after having killed him to free my self... he was actually very proud of me and ready to be my friend... as you should be friend with your familiar... even if I don't see how you can be a real friend with a demon for exemple...

You can be friendly with a demon but it might cost you alot more than your willing to give!.

The Ars Magica rules don't actually say very much about the relationship between Hermetic Magic and the soul.

The Limit of the Soul, as per AM5 pg.80, is written oddly. It states that "Hermetic magic cannot create an immortal soul, and so may not create human life" but then goes on with "nor restore the dead to life". The latter doesn't obviously follow from the first, since the souls in question presumably exist.

One possible extrapolation is that Hermetic Magic cannot affect the soul in any way. I like this variant because it keeps some mysteries of the universe beyond the ken of magi, in particular mysteries of salvation and eternity. There's nothing I know of in the rules that directly supports this though. Perhaps Magi can do all sorts of things with souls, just not create them or raise the souled dead.

With regard to familiar bonding, there's no implication that having a soul provides any sort of immunity to other forms of Hermetic Magic, so why would it affect this? Vanilla familiar bonding is restricted to beasts, so a breakthrough would be required to affect humans. Like all breakthroughs, the troupe should decide if it's atmospheric before allowing the required research to succeed.

ArM5, p.80, and A&A, p.31, clearly establish demons (and angels) have souls. We accordingly can also conclude that you can bind as familiars beings with "souls", because you certainly can canonically bind demons.

I always thought that the purpose of binding a Familiar was to broaden your power by artificially linking to something you are not.

A Hermetic magus can be said to have the following properties:

  1. a corporeal body
  2. a soul

Animals, mundane and magical have a corporeal body and no soul. Like a faerie. Hence potentially available as a Familiar, if you have the correct virtue to link to them. (Hermetic Magus in the case of mundane/magical animals)
Angels and Demons have a soul, but no corporeal body. Hence potentially available as a Familiar, if you have the correct virtue to link to them.
Spirits have neither a soul nor a corporeal body. Hence potentially available as a Familiar, if you have the correct virtue to link to them.

Of course, it may not be these two qualities that are important.
There are other properties that might be applicable. For example Cunning/Intelligence, or Realm affiliation.

And just to explicitly clarify, RoP:I, pg. 29:

Hence why Demon's Eternal Oblivion actually works as a permanent solution on demons, as opposed to the minor inconvenience the Divine version theoretically is.

Personally, I would have no problem with someone taking another human being (associated with the Magic realm - either via Might or supernatural virtue) as a familiar. However, it does expand the idea of familiar out a ways beyond that which is originally envisioned in the core rulebook.

Oh, and you can also bind human beings with magic - that happens all the time with Mentem magic (which also affects ghosts and demons); it's just htat the regular idea of "playing around with supernatural beasties" is Vim, which covers Mentem/Corpus/Animal for the magical set. However, it's the Mentem portion which allows contract binding, not the vim part.

If you look at Holy Magic, for example, the ability to bind a spirit to a contract is the same magic used to bind a human being to one, as well - Adjuration, in this case.

However, in thinking about it a bit more...probably not canon. As otherwise, the ability to take (for example) one's spouse as a familiar, for the simple cost of aspecting them with the magic realm somehow, would be mentioned somewhere.