It isn't charged per month, it is charged per week. It varies per complex (Vista is cheapest, then Chatham, then Patterson) and per apartment size (3/4 BR apartments are cheapest, then 1/2 BR apartments). Yes, the rent is taken out of the paycheck.
For roomies, you can either pick one through the roomie notification system, or get one randomly at check-in. If you find someone you want to be roomies with, you just enter their notification code at the wdwcp website.
Yes, the apartments are by gender (males room together, and females room together). They also go by age (21 and over, under 21).