The way I work is I have in my configuration a set of packages that I want to always have access to. Things like git, vim, gpg and so on.

Then for every dev project I have shell.nix defined which lists project's dependencies. I also have direnv installed, so when I enter that directory, the project's tools are automatically available for me. I rarely change the main configuration and more often project's shell.nix. And for one-off's I use nix-shell -p <package>