We've had one or two customers want to do this before, and it can be quite a struggle to get Salesforce to agree to the licensing. The customer portal is intended for customers and the view is that if your employees need access to Salesforce then they need the appropriate (full) user license. It is possible to get this signed off, but your circumstances need to be pretty exceptional (at least in my experience).
Chatter licenses are disconnected from customer portal licenses, so if you were using them together each of your users would need two sets of credentials.
In order to use knowledge you'd need either sales cloud licenses and the knowledge feature license or service cloud licenses. The full knowledge base functionality is available on Enterprise Edition as long as you have the appropriate user licenese.
You might want to look at creating a public knowledgebase and then locking down access to your corporate IP addresses - that way the knowledge information is available to your employees, but there is no authentication aspect and therefore no licensing. All you need are the appropriate licenses for the knowledge base authors/managers.