No confirmation on Jobs prohibiting cross-classing. All that was ever said in this regard is that switching to a job would limit one's ability to cross class.

Considering the recent confirmation that job quests will be unlocked by having a primary class at level 30 as well as a specific sub class at 15 it isn't a far stretch of the mind to say one will be further limited in the number of skills they can cross-class as well as only be able to take from the sub class.

As for the idea of materias granting skills, I don't think it'll be easy to balance or worth the trouble. The whole point is for jobs to be dependent on others. The limited cross-class is by design, and thus a system that covers that weakness is counter-productive.

Personally, I think a lot of unique passive effects should be incorporated into accessories as well as some rare gear.