I think I may have been looking at this magic system swap-out for Broken Spokes from the wrong angle. At its root, this is about making life easy for me and the players. Rather than learning the complicated interactions GURPS‘ magic colleges, spells and their prerequisites, a realms-based system should be about presenting a significantly shorter list of things to remember, with the decanic modifiers as an add-on that players can benefit from and learn as needed.
With the goal of making this easy for everyone in mind, is it really sensible to go about building a whole new array of realms, or welding something from Mage: the Ascension to the Cabal setting? After all, the spell colleges as written are actually kind of a cornerstone of the cosmology of the Four Realms, in that the decans are wellsprings of form that trickle down to the material world.
Even if I don’t necessarily agree with the divisions of the colleges — Protection and Warning? Really? — it’d certainly be easiest to take them as written. They’re already mapped to the decans for me, after all.
And in the spirit of that apocryphal quote from George Patton, why not just do it? At worst, I don’t like the way they play and then I move on to one of my alternate realm schemes, right?
Addendum: Saturday afternoon, I started putting a reference document together, pulling options together from across the pages of GURPS Thaumatology. It is more than a little dizzying. There’s a fair bit of flipping back and forth through the flexible magic chapter, referencing tables in the appendices — which I am not looking forward to recreating in a word processing document — and being distracted by potential options: “Hm, do I want to add the threshold limitation to this system? That could be fun.” I’m going to stick with keeping it simple for now; spending fatigue points ought to be a sufficient brake on the wackiness.