There was an interesting comment on today's GOTD -- a guy added win7 to a Windows 10 PC. It worked for him, & I can see situations where adding 7/8 makes sense, e.g. to keep using your GOTDs, but there are complications, & some of them may come back to haunt him [at least until the July update when 10 will be reinstalled anyway]. Here's how I think it should be done, with a maybe quicker but more uncertain alternative at the end. Bear in mind that the added version of Windows will have to be re-activated, & if it's the copy of Windows that came with a PC/laptop, you might not be able to.
The 1st step of course is to make sure you have a complete disk backup that you can restore -- that 2nd part isn't always easy but is critical if you want to be able to put things back. You also want to have the Windows 10 setup DVD or USB stick handy, along with your key, & that's a potential escape route if things don't go well.
For Windows 7 or 8/8.1 you'll either install fresh, or re-use a current copy of 7/8/8.1 with all your apps installed. Either way you should have a setup DVD or USB stick along with your key. If you're reusing a copy of Windows, you'll want at least a partition image backup of the system or Windows partition, plus similar backups of any other partitions if that's where your apps were installed. If you can, you also want a bootable USB stick or CD/DVD with an app like those from Paragon [there are a few brands or options now] for migrating the OS to different hardware. And you should grab a copy of the free EasyBCD.
Now, a new Windows 10 PC or laptop etc. will have the disk formatted for GPT -- an older win7/8/8.1 machine may be MBR. That shouldn't be a problem. You need to make room for Windows 7/8/8.1... You're probably going to shrink the Windows 10 partition, or you might be adding a hard drive to supplement the original, or replacing the original with something bigger. If you're adding a supplemental disk that's easiest, since you won't have to do anything to the existing disk. If you're moving to a bigger disk, restore a backup of the existing disk, or clone it, but do not use all of the space on the new drive -- you want to leave at least enough room for the added copy of Windows, & maybe for added partitions for apps or media etc. too. Afterwards, ideally running 10 from the original disk, create the partition where you'll want 7/8/8.1 & any other partitions while you're at it -- it's convenient to do this sort of thing with the new drive in a USB 3.0 dock.
If you're shrinking the Windows partition you might want to use an app like today's GOTD from IM-Magic, or use the one from EaseUS, or Paragon, or whatever you prefer, so you can see where the partitions are located on the win10 device -- I've done enough installs of 10 now that I can safely say it can put the 4 required partitions in any order & still work. Of those 4 win10 partitions ideally the 1st will be the FAT32 boot partition UEFI bios & GPT disks need to boot Windows - 2nd will be the Microsoft Reserved partition - 3rd will be the Windows partition - 4th, the Recovery partition. It's safest if those partitions are not moved, whatever their order on disk, & by that I mean less chance of speed bumps, but it isn't absolutely critical, so you can arrange things if or as you want [more on that later].
What you need to do next is run a defrag to pack the files at the beginning of the partition, shrink the partition to make room for the other copy of Windows, & create a new partition in that now available space. If there's nothing after the Windows partition it's maybe easiest, but it's not hard to make sure the new partition fits between the Windows partition & whatever partition follows it.
Now either install 7/8/8.1 fresh to the new partition or drive, or restore the backup of the Windows partition from your other machine to that new partition. If you restored the backup you'll need to migrate the OS, & here things get a bit iffy -- the older version of Windows may just work, may be easily migrated, may need a repair install, may need some work updating drivers etc. afterward, & I've seen all of the above one time or another. To see if it works, boot into 10, run EasyBCD to add the old copy of Windows to the boot menu, & give it a try. If it doesn't work, & you have a migration tool on a bootable USB stick/CD/DVD, boot to that & try it. If that doesn't work, boot to the setup media for that version of Windows, only do a Repair install -- Google for exact directions for your version of Windows. Afterwards check Windows Update, as well as [re]installing drivers for the new hardware. Check Windows Device Mgr. for any hardware that isn't working.
You can also use Device Mgr. to update drivers, but be careful -- Windows only knows your hardware by what drivers are installed, so it's completely possible it'll just install a newer version of a wrong driver. If you're in doubt, check Device Mgr. in 10, noting what drivers it uses -- in most cases they'll be the same in 7/8/8.1 & 10, & in fact, if you copy the C:\Windows\ System32\ DriverStore\ FileRepository folder while in 10 somewhere else, you'll *probably* have all the drivers you'll need, though you might have to force their use using Device Mgr.
If you installed 7/8/8.1 fresh, once you've updated everything including drivers, it's best I think to reinstall 10. If you want to move the existing partitions around, it's likely easiest to just delete all but the Windows partition 1st & now reinstall 10. As long as your existing copy of 10 is there on the partition where you're installing 10, it should let you keep all your existing apps & settings, at the same time making sure the 3 other needed partitions are in place. If you want to get more tech & create those partitions beforehand, microsoft[.]com has directions on how to do it using the CLI [Command Line Interface] DiskPart -- it's not something you'd want to do in a partitioning app.
The reasons for [re]installing 10... the biggest one is that if you wait until the update expected this July, you'll have to reinstall 10 anyway. 2) if you downloaded an ISO [regular or Insider build] today, you'll have a newer version of 10 than if you installed the 1511 build that was the last general release ISO. 3) if you deleted partitions as above, this should replace them. 4) 10 uses different boot files, & the files that support the different boot modes, e.g. Safe Mode, are different. If you installed 7/8/8.1, there's no guarantees those are still in place, weren't replaced by earlier versions, & will still work. Now you can put those files from 10 in place manually -- I've done it -- but we're talking some arcane stuff that easily would take you longer than re-installing 10.
Finally, a last quick wrinkle for you... you might be able to get your old copy of Windows working with a bit less work if you A) create a VHD in 10, B) restore your backup of the old version of Windows to that VHD, & C) use EasyBCD to add that VHD to the boot menu. It'll *probably* work, but no guarantees -- you *might* also have to do a bit of work to get it working again after updating Windows 10 builds, assuming the VHD you used is on the Windows partition [I've no idea what the update process would do with it, e.g. place it in the Windows.old folder?]. The more common suggestion is to use an older version of Windows in a VM, & it may indeed work for you. Whether you restore a backup of your old copy of Windows to a VHD added to the boot menu, or to or for a VM, the migration process is just the same as for regular hardware. Going the VM route however, in my experience the caveats, along with the odds of you being unhappy with the results, are high in number.