The Path to 60 functions on all classes that are under level 60 - as soon as you hit 60 you no longer get the buff on that class/job but will continue to get it for all other classes/jobs that are not yet 60.

Best thing to do is get the Stormblood expansion (it also contains Havensward) and carry on to 60 on your main class then switch over if you want.

Also - it is not known how long this buff will be active for so I strongly advise you to make the most of it. The amount of xp needed from 50 to 60 is considerable and the buff really helps.