1: You will need gladiator or marauder at 50 to be a tank at endgame of course, but it does not matter if you START as one. What you start out influences startlingly little.

2: Honestly, it really doesn't matter. You can go do the other classes at any time, and technically their stories advance to 50 with the job quests. Leveling all your classes will give you more versatility in terms of what you can play as for low level content and what cross-class abilities you have, but getting one job to 50 ASAP is useful for unlocking content that may come in handy later, as well as working on daily/weekly things.