Yes, you can use your DC as a RDP Licensing server. I do so myself. However, after a while, I started thinking I should have made it a single purpose VM instead but not because of any problem, just a foreseeable future where it ties in this function into my DC. But I have no problem with this set up particularly.
So my recommendation is that while it is possible, I say keep each server doing one thing as much as possible. A reference on this (other than my experience) is the book Practice of System and Network Administration.
As for DC and being RDP - that was not the object of your question but you mentioned it. At least on 2008R2, while it is not recommended (and I would avoid it as much as possible), it is doable on a small set up and it is not a problem functionally (I took care of a few sites in such a set up). But I would stay away from doing that.