If your new motherboard does not have any good old-fashioned USB 2.0 ports brought out to the rear panel, you probably won't get the Windows 7 setup program to recognise a USB keyboard and a USB mouse, when they're plugged into the USB 3.0 ports.
If you don't have any USB 2.0 ports, try an old PS/2 (6-pin mini-DIN) keyboard and plug it into the one-and-only PS/2 port found on most motherboards. This still leaves you without a mouse during setup, which makes life very difficult.
If you have an ancient serial mouse and a COM port (9-way D-type) on the motherboard, you could try that. You'd then have a working PS/2 keyboard and a serial mouse to setup Windows 7 on a spare SSD.
If you find a forum where someone has managed to bodge drivers to work with Windows 7 on your motherboard's USB 3 chipsets, you could try "slipstreaming" these drivers into the Windows 7 ISO, then burn the modified ISO back to DVD or a USB memory stick.
https://superuser.com/questions/659638/windows-7-slipstream-drivers-to-the-install-dvd-iso
I still think it is much safer to install only one OS per SSD/HDD.
Best of luck - you'll probably need it.
P.S. I still have a large collection of 3.5in and 5.25in discs, plus a few old 3.0in and 8.0in floppy discs.