To answer your question: no guideline (as is typical with nix).

I always check https://search.nixos.org/options first. There's a chance it's a package which requires setup or even a service that has extra config. If it's not there, then https://search.nixos.org/packages is next.

When multiple come up, it depends which prefix or suffix they have. Prefixes like pythonPackages. either mean the package is written in a specific programming language with its own packaging intricacies and its easier to keep them under that prefix, other prefixes like neovimPackages. mean it's a package for a program and something like a plugin or so. Probably more prefixes exist.

There are some agreed upon but badly documented suffixes (the usual nix style). But if it's a version suffix, then it's up to you to decide which one to use. In a comment you mentioned julia, well that's a programming language so you have to choose the version you want. Sometimes the manual has information on how to use the programming language or one of the wikis.

Anti Commercial-AI license