chatgpt's take on using packages of different versions in nix.shell (which seems to be what niv+nix-flakes solve too)

shell.nix

{ pkgs ? import <nixpkgs> {} }:

pkgs.mkShell {
  buildInputs = [
    # Packages from the default nixpkgs channel
    pkgs.python
    pkgs.vim

    # Packages from another channel
    (import (builtins.fetchTarball {
      url = "https://example.com/another-channel/archive.tar.gz";
      sha256 = "...";
    })).somePackage
  ];
}