the way to get a ninja to learn double sword is:
get a ninja
equip it with a nosadu,
go to pick abillities, on the 4th on down, click on that then pick double sword
your ninja will now start learning double sword + u can equip it with another sword now
helpful hint: if u are just getting the double sword abillity and ignoring the other abillites, you can equip another nosadu to speed it up.
To get gladiator:
you need a bangaa with 2 warrior abilllites
To get Gunner:
You need a moogle with 1 animist abillity