Is there a particular rover or ranger you are thinking of? I am trying to find a ranger which (a) allows railcard discounts and then (b) has a discounted price of under £12...
Edit: The 'Cumbrian Coast Day Ranger', for example, costs £18. National Rail Enquiries makes no mention of the restrictions imposed on Senior, 16-25 or Disabled Persons railcards, but does say that
In addition Family & Friends Railcard discounts 34% adult and 81% child are permitted subject to the conditions of the Railcard being met.
This isn't much help in the case of the minimum fare of £12, so I looked at the Terms & Conditions for the 16-25 railcard:
However, for all journeys made at or before 10 am Monday to Friday a minimum fare applies. (The exceptions are when you travel at weekends, on Public Holidays, or during July and August).
Even though you are using a rover or ranger, you are still making a journey. I believe that the restrictions do still apply.