Many Advantages can have Limitations, which reduce the point cost of the Advantage in return for limiting how, when, or on what it can be used (for instange, Magery can have Aspected, One College, etc.). I'm not where I can access my rule books at present, but it just occurred to me to ask: can I apply a limitation (such as "Magic Only", as already found on Wild Talent) on Extra Fatigue?
The underlying idea is to create a very small (SM -2 at least) Mage who can still cast large spells. I've seen this done with Pixies (in 3rd Ed., don't know if it crossed over to 4th), but I don't recall how it was done.