Well, any magic that "fits its world" is obviously OK --in fact, the contrary would obviously not be OK. Truth to be told, even though we all use the RM2/C/X magic system, I've always wondered how many of us "strictly" adhere to it. I know I don't.
A companion about how to fit RM2/C/X magic system to different magical paradigms could be interesting.
Magic Theory Companion? Paradigms of Magic?