You gain access to *ANY* Crossclass ability you have unlocked in a valid required or optional class. So, yeah, if you level Con to 34, but your Gladiator is only 30, you could be Pld with Stoneskin at level 30.
But, don't attach level directly to Jobs, Jobs can gain abilities based on their Class Level, but Jobs do not have levels on their own. They take on the level of the main class required by the Job.
So, A level 30 Gladiator can be Pld, and they would basically be a Pld with level 30 Gladiator abilities. When Gladiator reaches 40, you can now perform the level 40 Paladin quest (as your class level is high enough to qualify you), but you must be on Paladin, you can not initiate it by simply being on Gladiator, you must be on Paladin.
It may seem complicated at first, but it is sort of like Physical levels in Final Fantasy 3 (not 6), or like completing the Cave of Trials? in 4. But in this case, you work on leveling your Class, you work on gaining specialization with your Jobs.
For example, if you are on Thm, and you have Con leveled, you can equip cure and stoneskin on your thm. However, if you switch to Blm, you can no longer use any Con abilities (as Blm does not have Con as a required or optional class).
This means that Thm is more Dynamic, while black mage is more focused. You gain your Black Mage abilities, Thm abilities, Pug Cross-class abilities, and I always forget the optional Cross class, but it is not con.
I hope that makes more sense.
And yes, you have to level Mrd to the level of the abilities you want for it.
This better reveals the leveling mechanics and requirements.
In the case of Sch and Smn, it depends on the cross classes, but this could result in ALOT of leveling still required even though you could unlock them both by getting ACN to 30 (You still need to get the Required class to 15 for each, and level up each cross class to the level you want abilities for).
Just thinking that two jobs from the same class is a gimme is not quite as one thinks.
- Sometimes I use the word "so" too much...
Edited, Jul 2nd 2013 1:37pm by rfolkker