Hi OdinSon, welcome to TF!
Early and late, and my wife complains I fall asleep in front of the TV broadcasting the same old drivel. Its my snoring that she objects to most, she says. I need to drown out the TV so I can fall asleep, so I can stay awake at night, playing in peace with the PCs I suppose:)
The first thing with multibooting troubleshooting is to identify the drive with the active partition is the same one that is the first boot option in BIOS and only one Active partition per PC is a good rule, which is why I don't swap drives, if I ever do move a drive from one PC to another, it leads to madness!
If that's difficult, just remove all but one drive, on a single connection to the motherboard, usually sata these days, and set the BIOS to boot from that. I was born too close to the bronze age to have come to terms with UEFI and GPT - legacy MBR is what dual booting is all about, since some OS you may want to include is also ignorant of modern EFI BIOS methods. Then you can swap HDDs onto the same cable connector until you find a booting candidate - you may need to enter the boot menu between each disk swap so that the settings can be saved. After that you can add other HDDs to the system. I now have a selection of 500GB disks from old SKY + HD Set top boxes (cost me nothing) which contain a library of systems installed on them.
It's always wise to have some other media to boot the PC with - a live linux CD, or a WinPE with a graphical interface is good. Even a Windows 7 setup disk is better than nothing.
Windows 10 Recovery Tools - Bootable Rescue Disk is a great system on a USB stick or DVD to have - you almost don't need Windows 10!
A windows to go USB is even better - I have a windows 8.0 CP (consumer preview) Sandisk 16GB USB, which times out after a couple of hours since it is long expired, but it's lovely to use on almost any system I plug it into - and I know to set the BIOS to boot from the hard disk called Sandisk Cruzer.
Once you have an active system drive with Bootloader, bootmanager and the BCD store on it you are set to go. I consider the installation that it boots up to as a "Technician" PC, and all others booting from the menu are individual lab PCs.
I'd create a 30GB partition for a windows 10 installation , 20 for 8, and perhaps 15 for 7, 10 for XP and so on - slightly larger for 64-bit installs. They may be imaged, compressed, transferred and expanded later, as needed, or converted to VHD (or other virtual disk type) files if you are fond of highly transportable single-file OS containers, or wish to use them in VMs.
I'll stop here, I'm sure you have plenty to digest, and some questions for later.