TFS 2015 CALs

Hello guys,

I have been trying to understand TFS CALs for a few days but I still have some queries that I think I need confirmation about:

1. I have an on-premise TFS 2015 server and some VS Enterprise MSDN. All the users with MSDN subscription are added in the advanced user access level group in the TFS. This TFS server is also apart of MSDN subscription so no retail purchase. I need to know if I can add 5 free Basic users that comes with TFS server to my MSDN based TFS. These 5 users don't have MSDN subscription.
2. If I need to get more TFS 2015 CALs for my users without MSDN subscription, how do I do it? These users also require Microsoft Test Manager.
3. Can I use a Visual Studio Team Service TFS CAL with my on-premise TFS Server? And if yes, how will that work? Do I still get first 5 free users if I am going for TFS CALs only with VS Team Service?
4. Team Manager Extension, is it different from Microsoft Team Manager? How? Does Basic user can use Team Manager with on-premise TFS?

A swift response will be highly appreciated as we are currently migrating from VSS to TFS and these licenses seem to have stifled our efforts so far.

Regards

Shahzad