Windows

That is correct. You are limited to 75 licenses. You can have additional Windows 2003 servers in the environment. No, you may not use additional domain controllers to evade the 75 license limit.

If you need more than 75 user or device client access licenses (CAL), you need to deploy Windows 2003 server.

This FAQ may assist you in seeing the limitations built into SBS:

http://www.microsoft.com/windowsserver2003/sbs/techinfo/overview/generalfaq.mspx

The real note here is that this is a server designed for small offices. It is unfortunately used by a lot of organizations as a cheap way to deploy a Microsoft infrastructure, which when they grow, turns around and bites them in the keister.

Yes. You can have additional servers in a SBS 2003 environment, and you can deploy additional DC's in this environment. SBS 2003 only restricts that it be the Root DC.

As for the 75 user/device restriction, MS has provided the Transition Pack to remove those restrictions for organizations that may find that as a problem. I actually advise my clients to purchase SBS 2003 for the feature/cost ratio and add the Transition Pack if needed.

You can add servers just like a standard domain but you can only have one DC.