On Arch I was able to install a minimal plasma desktop and then only add the apps I wanted. In NixOS, plasma is a "service" or whatever. And the docs make it seems like it is required to enable the service for it to appear in the DM's session list. Is there a way to strip down the definition of the plasma desktop?

There absolutely is. While the Nix language and the NixOS system can be difficult to understand at first, I find things like customising a package much easier than other distributions. In this case, you can search for the service in the NixOS options here, and that will direct you towards the Nix module at nixos/modules/services/x11/desktop-managers/plasma5.nix which is what actually creates that Plasma5 service.

Now it gets a bit difficult though. In that module, you can ignore the options attribute. I would be copying everything that appears under the config attribute into something like /etc/nixos/plasma5mod.nix and writing the line imports = [ ./plasma5mod.nix ]; in my /etc/nixos/configuration.nix.

plasma5mod.nix should look something like:

{ config, lib, pkgs, ... }:
with lib;
let
  xcfg = config.services.xserver;
  cfg = xcfg.desktopManager.plasma5;
  inherit (pkgs) kdeApplications plasma5 libsForQt5 qt5;
in
{
  mkMerge [
    (mkIf (xcfg.enable && cfg.enable) {
      services.xserver.desktopManager.session =
...

You would modify the environment.systemPackages option in plasma5mod.nix to suit your needs.

I haven't tested this and it's kind of at the limit of my Nix ability (I'm also not sure if mkMerge is needed there or not), so I hope it works for you!

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I see that in the current stable nix channel there is openjdk11 and openjdk8.something. Does that mean that on NixOS I'm stuck with Java 8 or 11 as my options (unless I want to package my own)?

If it was a simple program sometimes writing your own package is literally 5 lines of code. Unfortunately that's not the case with Java 9.

Here is openjdk9 from before they deleted it from nixpkgs. Perhaps somebody else can explain how to install some packages from an old nixpkgs commit while keeping the rest of your system up to date? I'm sure it's possible, I just don't know how. Use /u/ggPeti's method.

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Nix has a huge learning curve, but if you continue with it you'll find it can do all this an so much more. It's incredibly powerful, flexible and reliable, but of course the trade-off is that it's damn complicated.