Does it make sense to keep two of the RX's with the Nvidias to have an 8 card rig or because I paid too much, it wouldn't.

Are you in the US? If you are, there's a few RX 580 cards available from newegg for $350 each.
https://www.newegg.com/Product/Product.aspx?Item=N82E16814202280&cm_re=rx_580-_-14-202-280-_-Product Nitro 580/8 for $360.
https://www.newegg.com/Product/Product.aspx?Item=N82E16814150794&cm_re=rx_580-_-14-150-794-_-Product XFX 580/8 for $330.
https://www.newegg.com/Product/Product.aspx?Item=N82E16814131713&cm_re=rx_580-_-14-131-713-_-Product Red Devil 580/8 for $350.

Just return the overpriced stuff you bought for $500 each and buy some of these if you still want to own some Polaris cards. It's still not cheap, but a lot better than $500. I've got all of these and both Sapphire and Powercolor have good coolers. XFX is not so good but still fine.

Thanks for that. I am in the US.