Requirements to make an aran?

Silly question, but does that mean that max char slots will be 18 (6 Adventurers, 6 Cygnus, 6 Aran), or does the Aran count as one of the others?

Uh.. what.

A character is a character, no matter what it is.
You have 4 slots to work with, 5 after Aran (assuming no character slots have been bought). There is a max of 12 characters right now, presumably 15 after Aran. Therefore you could theoretically have 15 Adventurers, or 15 Cygnuses, or 15 Arans on one server on one account. The combination doesn't matter.