How do I configure multiple nixpkgs's instances in flake's

[waffle8946]

@Jump1n

will packages from pkgs.stable also allow unfree packages as set in

No. See step #3 here for your choices, ultimately you have to manually import rather than using legacyPackages:

Mixing stable and unstable packages on flake-based NixOS system

Never set pkgs directly in the NixOS module system, as the other answers are indicating. Here’s a couple of concepts you need to know: If you want to access some inputs, or some other value across your config without creating an explicit option for it, use specialArgs or _module.args. (For things that you import, you must pass them in specialArgs specifically to avoid infrec.) Changing pkgs requires an overlay; however, for this application, you may not necessarily need one if you are willing…

But generally I wouldn’t recommend mixing stable and unstable.