So... on mixing and matching cultivation styles.... Internal Power (Odr, Zeal, Henki) is locked. You get one, and once you have that one, you can't get any others. It looks like Magic Stat (Fylgia, Haltija). I think I recall seeing the Christian magic stat somewhere, but I can't find it now.
It looks like External power is not locked, given that Sten has both Orthstirr and Vaki, and it looks like those are equivalent. Christian is Fervor. So... there probably isn't a whole lot of benefit to getting rid of Orthstirr.
I think you have to have one secondary to the other. Like, I suspect a true Finnish Cultivator has a lot more than 80 Vaki, with Sten's limited by his reliance on Orthstirr instead, but that they'd also need to spend that Vaki on all the stuff he uses Orthstirr for. Which works out fine if your Orthstirr is good, but means you'd be better off getting rid of Orthstirr and making a full switch if your orthstirr is crippled like Horra's probably is.
I'd guess that the combat-mode awesome (Frenzy, Focus, Rhythm) is similarly not directly locked, but that it's generally going to require whatever the internal cultivation is. Interestingly, shapecrafters mean that Viking frenzy doesn't, but it's unclear what that would cost for other cultivation styles.
I think they probably are locked. Remember you need your fylgja to be unrevealed to learn Finnish Magic, so other lockouts other than actually having Odr are also possible, and I'll bet Frenzy is one of them.
- It looks like Rhythm is actually the equivalent of shapeshifting slots? except they stash it in their magic stat rather than their phsyical stat?
I don't think it's exactly the same being more like hanging spells, but yeah, they seem magic-based...I think Sten may still have shapeshifting as well, so I'm not sure if they're direct equivalents...hard to say.
...and I don't actually know what Itse is.
Itse is probably the Fylgja equivalent (as it basically means 'self', which is what the Fylgja is). Which means Haltija, as the magic stat, is something else entirely. Which may imply that there's a magic stat for Norse Cultivation as well, likely locked behind learning Seidr if so.
I also suspect Rhythm and the hanging spells are Itse based, while efficacy with spells is Haltija, but that's a lot more speculative.
Or maybe Haltija replaces Fylgja and Itse is the undiscovered 'Power' stat for how advanced one's Cultivation is within their current Realm? That would also make some sense, though I dunno how we'd discover ours then.