Did you try it? It would take as much time to create a test project as it does to type the question.
I don't k now what you are asking. You ask about "should I require that my form-within-a-user-control" but it is not clear why you might need that.
If I understand what you are asking then it works except the modeless form remains shown when I close the form that showed it. So I assume you would want to explicitly close the modeless form.
First try it and then if you have questions then you can be more specific.
Sam Hobbs
SimpleSamples.Info