Having been to Guam, I think they could handle security quite easily. Easier in fact than New York or London. Since its Guam, they wouldn't have to worry about crowds as much. How many protestors are going to fly to Guam to protest? And for those that do, they are a US territory, and would get assistance from the US government. The FBI, the Secret Service, and the US Coast Guard, Navy, and the Guam national guard (1700 members) would likely handle most aspects of security. Points of entry are relatively few, so its easier to know who is coming in and out. Very little land to watch, and not much in the way of air traffic except for aircraft actually coming to or leaving Guam. Not much traffic, so no traffic jams to worry about.
Exactly. Guam couldn't handle the security. They'd have to bring in a largely military force for security whereas NYC could do the job with their civilian police units. Add on the fact that the NYPD is both one of the world's greatest crowd control details AND one of the elite anti-terrorism units, it's a no brainer.