Extreme difficulty dualbooting Windows 7 x64 on GPT drive

Solved:
Enabled Legacy, changed boot order of legacy and booted without doing ESC > F9
Some claimed that my BIOS is incompatible with Windows 7, which is obviously bullcrap :)
Anyways, I'm installing Windows 7 right now :)
Btw it gave me an error about the partition table not being in the correct order or layout or something, looked it up and somebody just pressed OK and no partitions were lost, so I did the same


I have:
-HP - Compaq 15 laptop
-Windows 10 Home x64 (that I want to keep, and have it dualboot with Win7)
-Intel Celeron N2840
-Unknown Motherboard
-F.31 BIOS, whatever that means
-8GB RAM
-Hard drive & partitions:
1TB GTP-partitioned HGST HTS541010A9E680
Partitions:
-650mb healthy unlabeled recovery partition (100% free)
-260mb healthy unlabeled efi system partition (100% free)
-835GB "Windows (C:)" healthy boot page_file crash_dump primary NTFS (83% free, Windows 10, want to keep)
-74GB "Windows 7 (W:)" healthy primary NTFS (100%, reserved by me for Windows 7)
-855mb healthy unlabeled recovery partition (100% free)
-21.25GB "Recovery (D:)" healthy oem partition (11% free, Windows 8 HP backup probably, want to keep)

-a Windows All-in-one ISO
-GPT-partitioned hard drive
-Disabled Secure Boot
-Disabled Legacy Mode in BIOS
-Set USB 3.0 in BIOS to Auto

I flashed the Win7 ISO onto my pendrive using the Windows 7 Download tool.
I disabled Secure Boot in order to load into it.
It was missing from the boot list, so I enabled Legacy Support to boot into it.
Booted into the ISO from USB
I got an error when at the custom installation\partition management screen about not being able to load the CD/DVD drivers or something, and a tutorial online led me to setting USB 3.0 to Auto, from Enabled.
I want to install the x64 version of Windows 7, alongside Windows 10.
The ISO booted up just fine, and I went through the Windows 7 installation process, but I could not install Windows onto my Windows partition (keep in mind I selected x64 Windows 7 Ultimate, some people say x32 Windows won't throw this error but I want x64) and when clicking on the details about why I cannot install onto that partition, I was told that the installation cannot proceed as this is a GPT-partitioned drive.
Keep in mind I want to preserve Windows 10 and all my files, therefore I won't clean the disk and repartition it.
Online I found sources which claim that in order to install x64 Win7 onto GPT I must run it in UEFI mode. I have acquired my Windows 10 boot file thing (whatever the name was, ended in fw.efi) from C:\Windows\Boot\Efi (I think) and I renamed it to bootx64.efi.
In Rufus, I selected the old windows 7 ISO (without /efi/boot/bootx64.efi) and when I chose "GPT on a UEFI system" or something along these lines, I was told that this is not possible as the ISO is not UEFI-bootable. Therefore, I extracted the ISO contents, inserted the bootx64.efi into /efi/boot/ (the boot folder I had to create) and then I created an ISO out of these files in MagicISO.
Rufus then allowed me to flash this ISO as GPT in UEFI system, and I quickformatted it, selected FAT32 and allowed it to have an extended label.
I attempted to boot into this ISO.
Without Legacy mode, I received an error (0x00000f?) stating that some file (something like system32\boot\...) could not be loaded. In Legacy mode, there were two options for the USB, so I chose the second one (as opposed to the first one which was UEFI USB) (both had the same name) which resulted in a screen with a semi-blurred font saying that no bootable media was found, press any key to continue.

I have previously managed to install Ubuntu 14 through this usb pendrive/flashdrive, with its own swap partition and ext3 partition without any problem. To delete Ubuntu I entered recovery, did 'bootrec /fixmbr' and deleted the Ubuntu partitions.

Some website also stated that I could loss-lessly convert my partition table from GPT to MBR, using MiniTool partition wizard, and when attempting to do so, I received the error that the system partition is on the drive I want to repartition and such action would render it unbootable.