video cards work on pure tty/console but one not in X/KDE
 

I have Slackware64 14.2 up-to-date stable except compiled a default 'custom' kernel 4.17.0 (and probably will use idlemoor's unofficial 4.17 SlackBuild) and a Radeon RX Vega and Geforce GT 420. I don't boot to X, and they work basically fine on the command-line with a monitor on the Radeon and two on the Geforce. Then when I start X, the ones on the Geforce stop displaying, and KDE doesn't have the Geforce ports listed as existent, let alone configurable. What do I need to do?

It seems X may now be as difficult as the old days when you had to configure everything. After some time of that, then for years it used to automatically configure dual monitors or cards with three different configuration tools. Then maybe 10+ years ago they discontinued those, but if you need a xorg.conf in this day & age (the theory is you don't) then how the heck are you supposed to configure anymore without those tools? I'm no expert at writing xorg.confs... but is that what I need in this case?

Apparently the Radeon doesn't even use the radeon driver, but the amdgpu one. The Geforce works on pure tty/console with nouveau driver but I read something about the 420 not working with some nvidia driver, whether that was for pure tty/console or X...

The currently correct Nvidia "long lived driver" for your card is version 390.77,

https://www.geforce.com/drivers/results/136120

You will need to blacklist the nouveau driver. The Nvidia driver can so that for you.

Apparently my Radeon RX Vega has four ports (three display-port, DP, one 'HDMI,' which I don't use) so I'm just getting connectors for DP-to-VGA (works) and DP-to-DVI. This should take care of it. I'm marking this as solved though it's not a solution to the original problem which people can discuss such things more here if anyone else has these problems, or they might want to start new threads.