How to install Windows 7 on a 2015 MacBook Pro?
I would recommend just installing Windows 7 as you would with Windows 8 +.

Substitutes:

Hold down the option key while turning on your computer instead of using refind, this will give you a boot menu.

Uncheck all the options in boot camp, except the option to install drivers to a usb stick.

Go to disk utility, make a new partition, and set it to MSDOS FAT.

Start up your Mac, booting from the usb stick(hold down option key while turning on your computer). When prompted for an install disk, choose the partition you made. Go through the rest of the install process, and then plug in your driver usb. Find the setup.exe, and go through the it's install process.

Then you should be all set.