Want to add Windows 7 to my Windows 10 computer so I can run some of my old games that are not compatible with 10. So far I've had bad luck. Install starts but then fails and I lose the bootloader for 10. Had to do recovery install of 10.
If you want to dual or more boot Windows OSes you need to install them from oldest to newest, otherwise you will break the system typically. The safer option would be to install Windows 7 as a VM in Hyper-V, VMWare player or other hyper visor.