1. I prefer full-ish servers. Would rather spend a minute or two in a queue at peak hours than have nobody to play with right away.

2. No. The stat differences are less than a single piece of gear provides.

3. No.

4. Not quite. You can level a marauder, and while you're a marauder you could take certain skills from any of the other base classes (conjurer, arcanist, gladiator, etc.). When you transform into warrior, however, you lose the ability to take most other classes' skills. Warrior, for example, can only pick from Gladiator and Pugilist skills, but never Summoner or even Arcanist. In fact, none of the advanced job skills (from Warrior, Summoner, White Mage, etc.) can be used by other classes at all.

5. Your build is determined by your job. As a marauder/warrior, you will be a tank. As a conjurer/white mage, you will heal. (Exceptions can really only be made for very talented/dedicated players with a full party/raid group that allows them to break that heavily-enforced archetype.)

6. Have fun and try a lot of things. Best part of the game for me is being able to do literally every class on a single character, so I always have a variety of ways I can play.