You don’t, it’s not necessary.
I’ve never used it and don’t plan to.
It’s a funny thing some people do: root on tmpfs. The idea is to minimise statefulness if you ever need to deploy from scratch, and you specifically opt in to the state you want to save. It’s not necessary.
Provide adequate space for whatever types of data go on each partition. You have to decide that.
What error? I don’t use swap either, but I never had such issues.
I would suggest not blindly following recommendations if you don’t understand why they were given or where they come from.
My personal approach to code (including with nix) is to keep things as simple as possible to serve the needs I face right now.