Job question

[Anova]

I didn't even realize there was an xp boost for leveling a class lower than another. That's good to know.

You get a 50% xp boost to FATEs and monster kills for any class lower than your highest one. For example, a Lv.15 pugilist would get the xp boost on a Lv.12 marauder until that marauder became 15 or higher.

Ideally I'd like to have two jobs on one character for flexibility when getting a party together. One DPS job and one high demand job like a tank or healer. I just wasn't sure if that was feasible. I also forgot about Hunting Logs as a source of XP.

It really is very feasible. Just so you know though, Tanks are in slightly higher demand compared to healers. Play what you like though. Some players like to cap both tanks, both healers, or a melee&ranged dps because the nuances make them slightly better for specific tactics/strategies they prefer.

About dungeons, is there a limit to how many you can do them per day like many MMOs? Or does that only apply to leves? I've only done the three story ones on the way to 20 once because getting a healer was a chore.

For most dungeons there is no cap. There are only two exceptions: Crystal Tower and Coil of Bahamut. Crystal Tower lets you run it as many times as you want, but you can only get one item per week from it. Most just run it several times until they get the piece they want. Coil of Bahamut is a 5-stage dungeon that only lets you run each stage once each week. Once you beat the boss for the stage, you move on to the next one and can't backtrack. These dungeons are the very last two pieces of endgame though, so you will not notice it right away. Fortunately, SE will be removing the weekly cap on the current stages of Coil once patch 2.2 is released. I can't say if they're doing anything to Crystal Tower.

Or, more simply lol: how important is the race you choose?

As said by Nutz, your race is purely for aesthetics and identity. The same stat differences are the same between an equally geared lalafell mage and a roedygan mage. It might affect things at early level, but at endgame where your primary stats are 400+, 2 or 4 points hardly matters. You literally only see a difference of 6 or 8 damage, which might as well just be from random number generators.