First of all, welcome to the server!
When you activate a non-default subrace your character gets booted while the changes are applied. Don't worry, you can log right back in again, you didn't screw up.
No limit to how many characters you can have, I'd be pretty screwed in that case (currently over 70 and counting...).
A few naysayers always grumble about "OP" characters, but there's no rule about it. As long as you don't take advantage of bugs or do something that severely contradicts lore (like a druid with an undead summon) you shouldn't limit yourself in that regard.
As for tips, Misty Potions (Ghostly Visage) are hugely beneficial at the lower levels, since the damage reduction offsets the tiny health pools.
Edit:
LibrisMortis_666 wrote:
Once you type "Drow" into your subrace upon character creation
Note that subraces are case-sensitive and should always be written in all lower-case letters (IE: "drow"), otherwise it doesn't work.