In practice, it's not too bad. ASRock motherboards, have "PS/2 Port 60/64 emulation", which makes your USB keyboard / mouse, connected through the xHCI controller, appear as PS/2 port hardware to the OS. (Or you can always use a REAL PS/2 keyboard, if you have one and the mobo has the ports, which most do still.)
My Asus H110M-A board doesn't have a UEFI option for it, but it appears to work anyways, it may be always on.
It's tricky to get the xHCI driver on there, because even if you install Win7 64-bit, in UEFI mode, off of a SATA DVD drive, and use a USB keyboard / mouse to navigate the installer, you still can't stick the driver on a USB drive and plug it in, because... no USB support.
So you either have to use the mobo CD/DVD, and hopefully it has the driver on there, and put it into the SATA DVD drive after installing the base OS, or...
You can also make a Rufus Bootable UEFI USB3.0 stick with Linux Mint 17.3 64-bit (I used Cinnamon), and after Win7 is installed, and the primary SSD formatted properly (by Win7), then plug in the USB stick, set up UEFI to boot off of it, and boot off of it.
The newest (17.3) version of Mint supports Skylake, and will let you download the drivers off the internet, onto the download directory in C:\Users\username , using Firefox in Linux, and after you finish downloading the drivers using Linux, then reboot, pull the USB stick, set up UEFI to boot off of the Windows Boot Manager on the SSD again, and then once inside Win7 64-bit, simply navigate to the Downloads directory, and install the various drivers, including LAN and xHCI (USB3.0).
That's how I've been installing my Win7 64-bit onto my Skylake platforms. It's a slight PITA, but not bad.
My ASRock Z170 Pro4S also has "Internet Driver Download" feature in the UEFI, which I tried to use at first. It asks for the storage device to put the drivers onto, and downloaded 128MB worth of stuff, but when I rebooted back into Windows, a search in File Explorer couldn't find the driver file that downloaded. I have no idea where it went to. So I had to resort to the Linux Mint 17.3 method, that I had used on my Asus H110M-A board.
