I want to say Siren population is probably about mid-ranged. I wouldn't freak out if you see few players in a major city. There are days when crowds are light, and days when it is heavy. Find the Odin, Behemoth, or Svara FATEs and you will see how quickly a massive amount of players will converge. What it does lack in player population though it makes up for with kindness. Just about everyone is friendly and helpful.

I don't PvP so i can't tell you much there, but there is no shortage of players who run PvE content. FFXIV is no different from most MMOs where DPS classes have longer queue times than tanks and healers, but gripes come more from people leaving once in a group rather than having troubles finding them.

You can definitely level everything on one character. The only limit is your inventory space, which once you hit level 50 will explode.

The glamour system is pretty awesome, it does have it's limit though. For instance, something that is specific for MNK can't be glamoured onto something that's specific for WAR. It also goes back to my second response, since inventory can quickly becomes an issue if you glam too much stuff. There's still some tweaks SE can do here to help making the glamour and inventory systems a little more manageable.

Welcome to Eorzea and to Siren! My name is Gemina, send me a /tell if you see me around.