Yeah, that’s what you want even though you’re not on an EFI system. A couple comments in the module definition indicate that this will let you generate a menu (suitable for loading from Arch’s GRUB installation) without installing anything to any partition’s boot record. Calling grub-install is skipped for every boot entry where the device is set to "nodev".
No, you don’t want to do that. Being able to choose which generation to boot into is an important feature of NixOS. You want to leave GRUB enabled so that NixOS still generates the menu that lists all of your available NixOS generations, so if you break something you can reboot into your previous config.
PS: I can’t remember the behavior, exactly, but I think nixos-install might try to save you from installing a system with no bootloader configured to be installed. If it does, you could try passing --no-bootloader (see the very short section on it in the nixos-install manual) to nixos-install. Only bother with that if nixos-install gives you trouble for without passing that flag. Let me know whether you end up needing that or not.