The issue really isn't with Bootcamp but with the Window's licensing
and activation. And all that depends on what version of Windows you
have and if you wish to continue to use Windows on the iMac.
If you want to retain Windows on the iMac, you will need to purchase
a license for the Mini. Also, depending on the Windows version and
how you obtained it, the license may not be transferable to the Mini
if you decide to remove it from the iMac.
So, step one would be to research your version of Windows
on the Microsoft site and determine what can and can't be
done in terms of activation.