The main reason I don't think NIN is likely to be locked to after level 50 is because it's coming to the game before the level cap is raised. Unlocking a new job when you're already at the end of the game, leaving you nowhere new to advance to with the new job, would feel rather pointless. The next set of new jobs, the ones coming in the first expansion, along with a level cap raise, would be the first place a level 50 unlock starts to make better sense from a gameplay perspective.

And from a lore perspective, I think they still have plenty of options to go either way while keeping a vibrant, interesting, and perfectly self-consistent story going.

So, the naming of referring to ROG & NIN as a "class" and "job" suggests levels 1 and 30. Gameplay works better assuming levels 1 and 30. And since we only know a teaser portion of the lore at this point, it could work with most anything depending on the rest of the story that we don't yet know. I'm going to assume the standard 1 & 30 unless we hear differently.

There will be time enough later for SE to add a more advanced version of "classes" (probably called something else) if they want to. It's a bit early for that yet. It also would be a bit odd for them to add the whole notion of an advanced class while only giving us one of them. When and if they do expand the class->job duo into a class->job->something else trio, I expect we'll be getting at least as many specialties in that third tier as we have now in the first two, which would also make it the sort of feature that belongs in a full expansion and not just an update.