I have need for some Window 7 machines. I purchase Window 10 machines, reformat the hard drive and then load Windows 7. Why make it more difficult than this?
Hardware compatibility. For instance, I recently replaced my motherboard with identically the same make and model, but I upgraded the processor from Skylake to Kaby Lake.
This was a dual-boot system, something I would recommend to the OP but with some caveats about convenience. After the hardware changes, I made a point of testing the system's behavior with Windows 7 as well as Windows 10. I hadn't used Windows 7 in the dual-boot configuration for as much as a year.
It boots into Windows 7 OK, but it keeps reminding me that it doesn't recognize the processor and that the processor upgrade reveals the age of the OS and MS's choice to no longer keep it current with successive Intel generations.
I have a lot of Win 7 software I run under Windows 10, and I know what it can be like to find out that this or that program or version won't work under the newer OS. Perhaps the biggest sense of outrage I felt occurred a long time ago when I invested $250 in the computer version of the Oxford English Dictionary. A change in operating system from Win XP to Windows 7 made it impossible to use that software, which was a very elegant computer presentation of the famous unabridged dictionary. The OED publishers never even offered an easy upgrade path, and -- in fact -- there was no upgrade. They just naturally assumed that since they'd provided the OED online, nobody would need (or want) the software for a single, local PC. [A**holes!]
Otherwise, moving from Win 7 to Win 10 I have been pretty lucky. I am still annoyed by pop-ups from this or that software developer asking if I want to upgrade to a newer version for north of $100 or more. This is a world I never imagined in the days when I'd write my own software to do complex statistical analyses, or provide an interactive program accessing a carefully-designed database.
Now, for the OP, I could suggest a dual-boot Windows configuration, so that boot time enters a presentation of both OS options selectable by choosing this or that box with the mouse. You either boot to Windows 10, or boot to Windows 7. You would have the different OSes installed on different drive volumes, and ideally, they would use different drive volumes and partitions, except perhaps some data disk with mutually accessible files.
The problem with that is the inconvenience of having to reboot in order to bring up the other operating system for such usage. So I would guess there is a solution with VM or something similar, and someone has already suggested an option.