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
The support is labeled "experimental" but it IS there.
The "experimental" support for GCN1 and 2 cards is for the open-source AMDGPU driver, which doesn't support AMD APP-SDK OpenCL for mining. The proprietary Catalyst drivers on Linux don't support the Polaris cards and the proprietary AMDGPU PRO drivers only support GCN3 and up cards. The Catalyst ReLive drivers on Windows do support GCN1 and up cards.
Uh, don't feed me that bullshit and tell me it's candy - you don't need the APP SDK, only the open AMDGPU driver and SOME libs from AMDGPU-PRO. I know because I've run my 290X side-by-side with my 480s/580s.
I tried to get it to work using the Catalyst drivers and open source AMDGPU driver on Arch Linux and wasn't able to get the Pitcarin GCN1 and RX 480 cards working together on the same rig. Besides the driver not suppoting both cards, It's also the Catalyst Pitcarin driver requires Xorg <1.18 while the AMDGPU Pro drivers requires Xorg >1.18 and downgrading the kernel.
https://wiki.archlinux.org/index.php/AMD_Catalyst#Xorg_repositories
If you have a way to get it to work I would appreciate it.