1. Almost anyone can learn how to use magic. However, one can't learn both magic and anyother form of energy (like ki attacks or chakra).
2.Unless they're old enough (like in their fifties or so) a character can learn only One style of magic.
3. There Can be more then one style of magic used. One just needs to write up the basis of how it works (like if it needs hand symbols or spell components) and the spells themselves.
4. All spells (learned and otherwise) need to be approved before use. On that note, a spell caster doesn't start off knowing the most powerful spells (unless approved before the character is made.).
5. A character who wishes to learn magic needs to have a master or a source (like a spell book) to learn how to use magic.