Like I said it's just best practice to not install something like a RDS server on a DC. There are not technical limits preventing you from doing it though. You're probably fine doing it in a small environment with 5-10 users. Obviously the license limitations for Foundation server are 15 users. Your RDS licenses are limited to the same and must be purchased in additional to the Foundation Server license. I would recommend using RDS Web Access deployment. That way you're not sharing the desktop of the server with your users.
http://technet.microsoft.com/en-us/library/dd647502(WS.10).aspxThe above link is all the information you need for installation and configuration.
MO