ISPConfig is a control panel. It controls these other services such as apache2, postfix etc.
The idea is, once installed you won't need to adjust any of the other services manually. The control panel does the lot through a friendly interface.
You can potentially install it on top of your previously installed services but as mentioned in the previous post, there are very good guides to installing the right services in the right order prior to ISPConfig as the final installation.
If you then uninstall it?
I think (and don't quote me) the individual services will work fine.