My NixOS install games as well as my Arch one does. Actually better than it since the latest Steam UI update.
I'd definitely recommend against starting with NixOS. Learning resources are way more sparse than something like Arch. Things like the division with flakes and the fact that everyone sets up their configuration differently means that finding examples that work in your config when you're starting out is even harder. Combined with learning how to setup a Linux system is too much for a beginner imo.
If OP is really interested in Nix they should install the Nix package manager on another working Linux installation so they can learn at their own pace.