Mysteries gives rules for immortal magi. Appendix A contains rules for magi trying to improve themselves, to learn.
All the paths to immortality work by changing the magus into a creature of magic - with a magic might score.
RoP: Magic contains rules for magic beings trying to improve themselves - are these paths/methods open to immortal magi as well?
I don't see why not, but perhaps I've overlooked something.
I would tend to say yes, though the immortal magi from TMRE have some other options. Usually you won't make yourself a really low MM immortal being, so it will take huge amounts of vis to counteract the learning penalties in RoP:M. With such large vis requirements, it probably won't won't happen much.
I seem to recall that the "official" answer and/or the intention of the authors is that indeed, yes, immortal magi can "learn" like other magical beings, though they also have other options available - as callen pointed out, their Magic Might tends to be high, which makes learning without extra vis virtually impossible, and vis costs to offset learning penalties staggering.
I would say "not necessarily" Generally, I'd limit the advancement options to those listed in TMRE, as that's how those rules were designed. I would, however, allow the RoP:M rules to be used for special occasions, to allow advancement that is otherwise impossible under TMRE rules or is in some other ways very special.
However, if it is a high-power saga (and if it's got immortal magi in it, it probably is), I'd just go by the RAW and allow RoPM advancement (I seem to vaguely recall that that's indeed the RAW, as ezzelino says).
A few things to bear in mind, if you're using RoP:M.
magic might gives powers. I think it gives too many, and I play a very high powered game. I make all powers cost double the official amount.
there is a power to suppress one's magic might, down to zero if you take it enough. While a 'normal' magic character would be very vulnerable without its might, a magus would still have parma; and magi also have spells so don't suffer as much from not being able to use the powers (and zero cost, constant powers still work). A magus would have to keep his might suppressed for the whole season to avoid the xp penalties.
As long as you know about this, you can decide in advance if you will permit this to work for xp purposes (it's intent, I think, is to allow spells to affect someone with might easily). For a transformed magus, I suggest not; for an actual 'magic' character, the problems from keeping one's might suppressed for the whole season are sufficient to allow it to work.
OK, I've just spent half an hour looking through RoP:M. It's one of the hardest books to find a specific power in I've ever read! It's possible that I made it up...or I just haven't spotted it. My character notes have: Personal Power, suppress might score and pool by 10 points. Duration sun, cancellable at will. Might pool has to regenerate normally. I've not noted down the cost, either free or subsumed in the 10 points.
The purpose of it is to allow (friendly) magi to cast spells on the magical might creature. Normally, you can't suppress magic resistance based on having a might score. It is not to allow the might creature to affect others, as you said, you need more penetration for that.
Regardless, powers can be invented and they can break hermetic limits, so even if I've invented it, it would not be unreasonable for it to exist as long as the SG knows whether to allow any such thing to affect xp penalties.
Are you sure about that? I haven't found specific mention either way in the last few minutes, but I have found a couple comments such as "functions exactly like Hermetic resistance," which would seem to imply to me that it can be lowered. However, regardless of whether it's allowed or not, I wouldn't let a non-intelligent being do so.
Yes, you could certainly invent a power to suppress Might. I just didn't think one was listed, which is why I asked. Still, if it were to apply to learning, I would expect it would then require a vis cost or else the learning would only last until the Might suppression ended or something like that. Otherwise that would basically destroy the whole process in RoP:M.
Here's an interesting concept: you have to lower your Might to remember your Ability. Extend it to you no longer care or remember others when you are strong and you have a nice story. I remember a science-fiction story which ended more-or-less in "Yes, I will attack you again, just hunt me until I become smart enough to remember we are friends."
Considering that knowledge is power and that learned XP transform into vis according to RoP:M, the real trick isn't in amassing vis, it's just in sneaking a decent Latin primer into the Magic Realm of your choice and reading it for a season.