Main Story Quest AND Changing Main Class

[Thaliss]

Guys thanks a lot for your answers and this was exactly my understanding of the main scenario but was not sure and indeed this was also what I was doing but takes far more long to pex for the moment .. So the main scenario sticks to the character and NOT to its main class, i.e. I will be able to do my main scenario with my MRD once I hit lv 18 with my MRD correct ? ..

Yes the main story resumes once the class you are currently on (in your case MRD) reaches the appropriate level.

That can be done fast to get to 18 with your hunting log, leves/fates and once 15 Tanking in dungeons for the gear as you will have fast queues in Duty Finder if you don't have a group already.