[indypartridge]

#1 I'm concerned about skunks. Is this something I prepare for beforehand or wait until I see signs of their visit to do something? What is the solution if a skunk is bothering the hive?

Wait and see if there's a problem. If it is a problem, the hives need to be elevated a bit - enough so that the ***** or possums have to stand up in order to reach the hive entrance (this exposes their undersides to stinging). Second, make it uncomfortable: carpet tack strips on the landing board won't bother the bees, but make it difficult for Mr. Skunk to find a place to put his paws to stand up. Some beeks put a bunch of tacks thru an old piece of carpet and put that in front of the hive.

Another, perhaps simpler method, is to use top entrances on your hives:
http://www.bushfarms.com/beestopentrance.htm

#2 If bees are robbing my bees hive, what should I do? What are the drawbacks of leaving the small opening in all summer, doesn't that help them from pest invasion (i.e. wasps, hornets?)

A strong colony should be able to manage against invaders. On occasion it's necessary to reduce the entrance of a weak hive to help it defend itself, but it's not something to leave in place. For a strong hive, a small entrance just creates a lot of congestion during a honey flow.

I don't live near where there are bears, what other invaders should I be aware of?

Obnoxious, rock-throwing children?