stefanos welcome to the Whonix forum!
Tor Browser in Whonix-Workstation is adjusted to work behind Whonix-Gateway to prevent Tor over Tor. So its safe to use Tor Browser in Whonix-Workstation.
You can use other operating systems along with Tor Browser behind Whonix-Gateway if you would like. However, the default is recommended if you are not an experienced Whonix user.
I would recommend sticking with the default Whonix-Workstation until you get some experience under your belt. That way you won’t have to worry about Tor over Tor scenarios.