Actually, if you run the AMDGPU-PRO drivers, you CAN mix them in a rig. The Fiji card may perform slightly worse than with fglrx, but it works.
Since AMDGPU Pro drivers only support GCN3 or newer cards, I couldn't get it to work with my Pitcarin and RX 480 rig on Arch Linux. I had to switch to Windows 7 to be able to use all my cards
https://wiki.archlinux.org/index.php/Xorg#AMD