Modules typically consists of server and client component. Client component is also called as plug-in. when you install server component, client component is available for download for vSphere client. When client component is installed interface of vCenter gets changed in the form of tabs and etc.Please note Modules needs to be upgraded independently of vCenter.
vCenter has built in database calculator, built on whatif scenario, which means no changes are made to DB.
You can deploy virtual center behind firewall but make sure there is no NAT between vCenter and Host it is managing.Before deploying make sure you understand hardware,software requirements of vCenter.