The Rich Text control is for when you want to have multiple formats or other Content Controls within. You can do that and set the control so that it can't be deleted.

You can require a certain style in a Plain Text Control. What I usually do is just insert the control and then apply what formatting I want to the entire control if I did want some special formatting. In my experience, such formatting works fine. I would use the CC property of a certain style only if I wanted it to keep that style no matter what the user wanted to do.