Windows 10 does not have any anti-pirate means against using a pre-activated version of Windows 7. As a matter of fact, in the early advertisements, Microsoft even mentioned that there is no issue using an unlicensed copy to upgrade and they wont give you any grief over it. The idea behind this was to compete with the open-source crowd by trying to make things easy for people who want a totally free OS.
However, the free upgrade end date is July 31, 2016. So be sure to take advantage of the free upgrade while it still exists.
The biggest issue I found using Windows 10 on the Mini was that Windows Defender causes random excessive slowdowns. So I tried disabling it, but it will automatically turn back on eventually, and you actually need to go into the registry to force it to stay off indefinitely. Otherwise, all the Dell drivers for Windows 7 works when installed under compatibility mode.