Configure a network with an AirPort Extreme and Mac Mini Server? I'm trying to set up a Mac Mini Server for Local File Sharing, Calendar, and Contacts, but I also want to be able to access everything remotely through a domain. I'm using an AirPort Extreme as my router, I also have a Time Capsule configured to back up the entire Mac Mini Server.

That said, I also want to have multiple configured users so that I can harness permissions so that all files aren't available to every user. Also I want to have 2 Administrators for the network.

I've been messing with the settings and thought I had it working, but there seems to be an issue now with permissions. Only admins can login and everyone else can't even connect to the server.

If it even matters, I also have a separate remote dedicated linux server and a domain already.

I'm tempted to start from scratch, but if I do, where do I start? Is "File Sharing" the correct approach? or should I use something else?
Just buy the Server app. It costs about $20 which is a real bargain. There's also a wonderful video tutorial on the server setup on YouTube by Todd Olthoff. I installed a server with about the same configuration as the one you are contemplating and it took me a while to complete.