Do Mystery virtues count as hermetic virtues for the "you can only have one major hermetic virtue" limit?
For example if one wanted to play a Tytalus who was a cabal legacy of the Titanoi, could they take both major magical focus in spirits, and the major mystery virtue Invocation Magic?
And what's a reasonable number of mystery virtues to have at gauntlet, if you do take Cabal Legacy?
One of my players (it's me. The player is me) wants to take Cabal Legacy along with all of the Hermetic Theurgy minor mystery virtues, justifying it as because they also cannot cast spontaneous spells, that's what their parens focused on.
The limit of one Major hermetic virtue at gauntlet still applies.
What's a reasonable number of mystery virtues to have at gauntlet? Whatever would be reasonable for someone with under fifteen years of service in your saga. If he got initiated a couple of years into apprenticeship, they could have got two or three minor virtues by now.
I generally think that at Gauntlet what makes sense is solely the first in the chain... Depending on the mystery, sometimes not even that.
To answer your other question, no, Mystery Virtues do not count as Hermetic for the purpose of the limit at character generation.
EDIT: Just to be clear when I say Mystery Virtue I mean those virtues categorized as Mystery Virtues, not just any virtue one gets initiated into via a mystery cult.
One major hermetic virtue is one major hermetic virtue. Doesn't matter where you got them from.
It is not so much a matter of number of mystery virtues, but how far he would have ascended the ranks in the mystery cult. A young magus is most unlikely to have obtained high rank in the cult, and thus won't have gone through many initiations.
But if you need a number, then one or at most two mystery virtues would be reasonable to have at gauntlet.
If and only if they are "intrinsically" Hermetic Virtues.
If your Mystery Cult initiates you ... e.g. as a Nightwalker, that does not count as a Hermetic Virtue.
If your Mystery Cult initiates you into a Gentle Gift, that does count.
Remember: "A magus with a Cabal Legacy was Initiated into an Esoteric Mystery Cult while still apprenticed, and may take cult Virtues and Ordeal Flaws as part of his normal limits." (TMRE p.7, emphasis mine).
In general, no.
However, if you do play out the apprenticeship, then at the time of the gauntlet the character may no longer be within the character creation limits: he may have multiple Major Hermetic Virtues, have Virtues and Flaws not balanced with each other, and have more or fewer xp than those of a "standard" character of his age (from my experience, he'll have more, but not nearly as optimized).
Also, if you create a magus who's not newly gauntleted, then you may (with troupe permission) use some of the advancement seasons for Initiations, though "as this rules out interesting stories, it is not encouraged for player characters" (see TMRE pp.19-20).
Anything that does not violate your normal character creation limits, as long as it's possible (e.g. an Initiation that requires you to have participated in some event every winter solstice for a century is probably off-limits for a 20-year old mage).
Sure, why not. That's just four minor Virtues if I am not mistaken: Names of Power, Hermetic Theurgy, Theurgic Spirit Familiar, and Hermetic Synthemata. No need to "justify" it.
By the way, remember that "the other half" of Cabal Legacy is that you can take Ordeal Flaws that might be unavailable to a vanilla magus.
Just to be consistent, don't forget to give him a decent Cult Lore score. As a guideline, Verdititus Lore of 3 is required for minor virtue and a score of 5 for major one.
It is an indication of his commitment and understanding of the Cult.
I know it is not required to have any score in Cult Lore to be initiated, but come on, who is gonna share cult secret with somebody who does not care, support or understand their cult ?
Except for the four Mystery Houses (and those are excepted only for backwards compatibility), it is actually required to have a score of at least 1 in the Cult Lore before going through your first mystery initiation. (TM:RE p10)
I generally agree with what @dc444 said above, but I do think it depends a lot on how you play the Cult, which may in turn depend on the particular Cult.
I prefer to play the Cults as secret societies with their own rules and expectations on the members. As others have implied, it takes time to advance the ranks, and service may be expected. Nobody would ever admit an apprentice beyond the first rank, and that only if it works more or less like a Mystery Cult House, where members are trained as apprentices.
Other cults may be more individualistic. Initiation is sold by mystagogues making a limit, and a parens may initiate their apprentices in several mysteries, just like any other kind of training.
I think you have to design the Cult and its customs to know what is reasonable in any particular case.
Game balance wise, I do not have an issue with allocating mystery virtues like other virtues, within the ten point limit. The concern is to make a plausible narrative, but we do not normally worry about wherer virtues come from. They are just part of the character's nature. It only really matters when the Cult features in the story.
I think there are cases where you get a special apprentice. If the apprentice's gift manifests with a major focus in spirit magic, for example, I could see a theurgist parens focusing on theurgy.