AIM Server for Linux
 

Is there a AIM server that I can run within a business without interacting with AIM's network. Or is there any other cross platform client/server chat software?

I haven't seen any AIM servers but I have seen jabber servers and they definitly have clients for Mac, Linux, and Windows.

http://open-im.net/en/index.xml

Does open-im provide a client or do I use MSN Messenger, AIM or Yahoo for the client and configure the clients?

Open-im is just a server. You need a client that supports Jabber in order to connect to it. There are literally dozens of clients that support it, just search google for one.

Is there any documentation on how to configure clients with OpenIM? It feels like I'm working in the dark here.

Jabber is a pretty decent platform for running your own IM service. It's pretty flexible and relatively secure. There are also plenty of clients like Exodus and PSI that work well.