So, I've got someone with Firaja, Blizzaja and Thundaja (sorry, I've been conditioned on the new translation ;P). When I cast them on other party members though, they don't learn it except for one occasion in which one party member did.
What's up with this? Is it only a chance to learn it or something?
Yep, it's the same chance that the spell would appear in the skills list after your character would turn into a crystal. On the top of my head those spells have 15-20%, Zodiac is 90%, Ultima is 100%.