If I join a community, and make a friends only or custom security post in my journal, can the other members of the community see my post?
No. If you add a community to your friends list, the only people able to see your "friends only" or "custom security" posts are the actual community account and any members of the community which you had already listed as your friend.
Update: You can't really tell, but that comma after "If you add a community to your friends list" is bold as well. It should be added in to make the sentence run more smoothly.