netia said:
What I remember from that time - it was essential the older OS had been installed BEFORE the newer one.
Is this rule (the order of installations) still important ? Looks like not really... Am I right ?
If you are going to install the Operating Systems on two separate hard drives, then it doesn't matter which one you do first. What matters is which one you boot into when you add the second OS to the boot menu. If you are going to install both Operating Systems on the same hard drive, in two different partitions, then you should install the oldest one first - but with Windows 10 and Windows 7, it isn't disastrous if you install Windows 10 first.

netia said:
Sorry, it might sound silly but I hope I won't be needing to go to UEFI each time I boot to determine what will get loaded. ....
You eliminate the need to do that by adding the second OS to the boot menu of the first OS.

netia said:
I have to use UEFI/GPT on 3TB drive (Win10), but what would you advice me to use on the second, much smaller (500 GB) HDD (Win7) - UEFI/GPT or BIOS/MBR and why ? (possible advantages / disadvantages).
Someone else might be more experienced, have better advice than me. If it were me, I think I would do BIOS/MBR on the second hard drive. I think it might be easier to install Windows 7 that away and work better adding it to the Windows 10 boot menu. The disadvantage would be that if you have to disconnect the first hard drive, your UEFI firmware will have to be set to legacy booting to boot from the Windows 7 hard drive - and you will have to set your UEFI firmware to boot in legacy mode to install Windows 7 in legacy mode. Legacy mode is another name for BIOS/MBR.

netia said:
Please note : my smaller drive is empty so I'll be able to install Windows7 on unallocated space there.
But it's not the case on 3TB HDD (Windows10), that is fully partitioned and need to stay that way.
Beside the main OS, it contains / stores huge amount of my (user) data (~~2TB).
Please check the screenshot below for reference (DM of my 3TB HDD).

[C partition wasn't yet renamed after upgrade to Windows7]
Your 3TB hard drive is set up very well for a clean install. You will want to select the custom install option in Windows 10 Setup, you will get a list of the partitions on the drive. You will want to delete all the partitions before the first data partition. There will probably be 4 of them because I think you will see a hidden MSR (Microsoft System Reserved) partition in there. Then pick the unallocated space that will result at the front of the disk to install to.

netia said:
I'm not sure what do you mean by "recovery partition" ? Hidden UEFI recovery partition, created automatically during any UEFI system installation ?

However re. Disk Management screenshot of my 3TB HDD from the previous post :
I think this 450MB hidden partition to the right of C:\ drive, that I never created myself - is just that UEFI recovery partition. Am I reight ?
Most probably it's been created during the recent Windows10 upgrade.
Could you or somebody else inform me what is it for ?
That is the recovery partition. Installs of the newer builds of Windows 10 in legacy mode seem to combine the recovery partition and the system partition (the system partition does not contain the Operating System, it contains the boot files). Installs of Windows 10 will always create separate EFI system partition (where the boot files are) and a recovery partition. I can't honestly remember what partitions a clean install of Windows 7 creates.