What security should I double-check before opening my site to being multi-user?
Mittenchops, here is a suggestion


Create a role for the new users
Guarantee that this role doesn`t have permission to use PHP Filter 
Check all the "administer " permissions to see if the new role is not
having access to certain areas that it should not have
Create a new user for you with this new role and test the site.
During the process of creating the new user check if you are able to choose the role (you shouldn't be)
You should also try to access pages like http://yousite.com/admin which a common user should'n have permission


I think you would be safe with this.