If you want stressed-out idealists many of whom haven't ever held a real job before, man you hit the jackpot! Otherwise it's probably a bit harder to find a relaxed mate. The Environmental Policy people seem a little more chill than the Terrorism Studies or Non-Proliferation students though.
Housing in Monterey is fairly nice--especially compared to Santa Cruz, on the other side of the bay--and not too hard to come by.On average, it's not cheap, but there are some decent affordable options. Contact the school or use its Facebook page for new students to find a tolerable roommate--you'll find something without a problem!
Parking can people a pain, and it seems like most commuter students who drive (and teachers) are constantly battling the time limits and moving their cars. However, MIIS is located in downtown Monterey and the surrounding area is very easy to navigate on foot/bike and public transportation is plentiful.