Yeah, exactly! For steam itself on NixOS you don’t have to manually use steam-run, but steam-run is a handy little tool to wrap / run other commands with the FHS that NixOS sets up for steam. I’ve mostly used it to run a few Linux games that I have binaries for, but don’t have on steam… I’m pretty sure I used it for another Linux program too, but I can’t remember what right now.
I’ve been having a great time with games on NixOS. Steam just works when you enable it. I believe you can specify extra libraries for the filesystem hierarchy hackery, but I haven’t needed to yet. One thing you should know about (if you don’t already) is steam-run which is a simple command line tool that automatically wraps things in a normal FHS. Super convenient for the occasional binary :).