Welcome! 
Have a look at this guide, if you haven’t yet:
You shouldn’t have to worry about the specific pieces of software that others mention, since they might just be asking for their own personal use cases, which may not align with yours.
“Hardening guides” are usually for general-purpose operating systems that don’t focus on security. Since Qubes is a security-oriented operating system, most of the “hardening” is already designed into the architecture of the system. I’d recommend against tinkering too much too quickly and suggest instead first understanding how the system is intended to be used. The rest of the documentation is great for this:
Since you mentioned that you already have Whonix, then no, you don’t have have to install Tor. Whonix already includes Tor. The Whonix project has their own documentation on using Whonix inside of Qubes: