Mixing release streams dont infect other packages but yes you will have multiples of a lot of dependencies. Thats the one thing nixos eats up pretty well but with modern storage it doesnt really matter. I have a 512gb ssd and only use about 200gb of it with nixos, vms and all my personal data. I havent thought about running out of storage in a lot of time. Tho your own usage may vary, i dont have many heavy packages other than a few design tools.
The zero time really is true, i just search the nix repo, add it to my config and rebuild. 2 mins. The only things ive struggled with are dependencies for building stuff but ive always been someone to struggle with it so i just started compiling in vms and now i just do it with distrobox. It works for me, i didnt even change my workflow from other distros but it may not work for you. I definitely recommend trying nixos out and if you like it you will love it. Just dont go too deep in the rabbit hole, i know people whos home is a temp directory and its rebuilt on every boot and their root is also rebuilt the same way. Dont even ask why or how 💀
People doing anything to not use mastodon…