Can I have these two gpus in a single rig?
I'm looking to have 4 rx 570s and 2 rx 470s
yeah, but you will need newer drivers, which could be a potential problem.
in windows, new drivers check signature, which is a problem if you bios-mod your GPUs.
16.9.2 driver (that does not check sig) will not work on 570ies
in linux, same, you will need 17.something amdgpu-pro drivers, 16.40 drivers would not work on 570/580.
other than above, it should be fine.