So how can I know which commands are available at the time of installation?

Commands available after a fresh install of Mavericks (OS X 10.9) belong to one of these four packages:

• com.apple.pkg.BSD
• com.apple.pkg.BaseSystemBinaries
• com.apple.pkg.BaseSystemResources
• com.apple.pkg.Essentials

(Note that, as of High Sierra (macOS 10.13), commands have moved to this package com.apple.pkg.Core.)

You can list the commands included in every package with this command:

pkgutil --files <package name> | egrep '^usr/s*bin|^s*bin/'

Are netcat, nc available by default on Mac OS X or is that installed along with command line tools?

I found nc with:

pkgutil --files com.apple.pkg.BaseSystemBinaries | egrep '^usr/bin/nc'

(On High Sierra, run pkgutil --files com.apple.pkg.Core | egrep '^usr/bin/nc' instead.)

so yes, nc belongs to the base OS installation.

I couldn't find netcat, so if you have it on your system it was installed later.

To list all commands provided by all packages, run in Terminal:

for p in $(pkgutil --packages); do 
  list_of_cmds=$(pkgutil --files $p | egrep '^usr/s*bin|^s*bin/')
  if [ ! -z "$list_of_cmds" ]; then
    echo ">>>> $p <<<<"
    echo "$list_of_cmds"
  fi
done

You can also pipe the command to a file on your Desktop for later reference:

for p in $(pkgutil --packages); do 
  list_of_cmds=$(pkgutil --files $p | egrep '^usr/s*bin|^s*bin/')
  if [ ! -z "$list_of_cmds" ]; then
    echo ">>>> $p <<<<"
    echo "$list_of_cmds"
  fi
done > ~/Desktop/cmds_from_pkgs.txt

Compass

Compass shouldn't require the Command Line Developer Tools, just a compatible ruby environment.

I suggest a ruby version tool instead of using the system ruby, so that you can control your ruby version/environment. I favor chruby, but any of these works well and can be installed without needing the Command Line Dev Tools:

• chruby
• rbenv
• RVM

Once you have a ruby env with the version you want you should be able to install Compass from the command line (as detailed on the compass site):

$ gem install compass

Note: the "$" isn't part of the command, just an indicator of the terminal prompt

Command Line Developer Tools

While the above works for Compass, it sounds like you are asking about general package management with OS X as well. Homebrew is a very popular tool for this, but depends on the Command Line Dev Tools. MacPorts is an alternative but has the same dependency. I'm not aware of a package manager for OS X that doesn't depend on the Command Line Dev Tools.

In case you haven't tried one of these methods to install the tools I'm throwing them out there:

1. Download directly from https://developer.apple.com/downloads/ (you need to login with Apple ID). The URLs provided there may be resumable if the download is interrupted, and may be more reliable than installing without Xcode via the terminal.

2. Install without Xcode via the terminal:

   $ xcode-select --install
   

Having the tools definitely makes things easier; I personally love Homebrew (and it's offspring Cask) for managing both CLI and GUI packages.