If you plan on leveling them all eventually, then the timing of when you level your first and when you branch out to others is almost entirely a matter of personal preference. The only two caveats are:

(1) you should get the needed secondary class to 15 by the time your main is 30 so you can unlock your job, and

(2) there are a couple of non-progression related features (specifically personal housing and glamours) that aren't unlocked until level 50, so if those are important to you, it might be a reason to try to hit 50 on at least one job sooner rather than later.

The rest of what gets unlocked at 50 (or actually by completing the original 2.0 story, which requires being level 50) is only pertinent to your level 50 job, so whether it comes before or after leveling other classes/jobs is irrelevant, as you won't be using any of it while leveling those other jobs.

(I can't say much regarding the Heavensward leveling from 50 to 60 as I haven't done that part myself. I don't see any reason to until finishing all the ARR content.)