We are running ArcGIS Enterprise 10.7.1 on Red Hat Enterprise Linux. It is federated with both Server and Portal on the same system. Web Adaptor is configured and working and is required because we will need web-tier authentication. Because Web Adaptor only works with ports 80 and 443 we had to disable Apache (httpd) to avoid conflicts with Tomcat.
How can we use one system to host our web apps and ArcGIS Enterprise?
To answer my own question, this is how we resolved the problem of hosting web pages and running web adaptor on the same Linux machine, in our case Red Hat Enterprise Linux 7.6:
Hopefully this helps others who have a similar situation. Good luck!