*I cannot guarantee this is correct, but I'll try.

I'm not a tank myself, but from some of the feedback I've read/heard it's somewhat difficult to tank lower levels as you
do not possess a lot of threat generating abilities I think until the 20s or so? It seems as though it's a bit more
challenging to hold threat. Not to the point of it being a nightmare, but you definitely have to pay attention.

I've seen more than once that healers can pull threat if they aren't careful, which is normal, but to me it seems
a little less lenient than say WoW was. Easier to lose your grip on an enemy. This gets a little easier once
you get higher up, as you get more abilities and such.

As for your second question, I assume you are asking how you level other classes and use their skills? If so,
once you hit level 10 you can go to other class guilds and "sign-up" per se, and to change to that class
you simply equip that classes weapon and bam, you are that class.

As to using their skills, each "job" can only cross-skill from specific classes. I forget which Paladin can do,
I believe it's Conjurer and... I'm spacing the other.

Hope this helped. Hopefully a full-time tanker can answer better for you. Again, if I'm wrong in any of this
I apologize. I've yet to touch a tanking class. >_<