So in Geddoe's place limiting to his canon game mechanics (i.e. four levels of spells ranging from "moderate" to "VERY STRONG" with a fixed number of times each spell can be used from say 7 of the weakest to 2-3 for the highest) would probably be fine?

In Sion's case he's a weird on in that he has basic normal human abilities to use magic... and then has an utter godmode ability, so that's the harder one to nail down. Although it's NOT LIKELY he'll use the latter unless he feels he absolutely needs to, I'd like to know in case it does come up...