The Express version of Oracle is not supported for the Pega 7 Platform.  You can check the Platform Support Guide for more information about supported databases and application servers.  Page 28 states: 

"Note: None of the “Express” or “Personal” versions of these databases have been tested with the Enterprise version of Pega 7; no express/personal version of a database is supported with Pega 7."

Frameworks are now called Industry Applications. Check the release note for your specific application to get more information about the system requirements for that application. See Products > Industry Solutions.

I understand that PEGA does not support PEGA 7 Platform when using Oracle XE but will it install and work?

Reason I ask is I am planning on setting up a development server and money is an issue so I plan on using CentOS+Oracle XE+Tomcat Apache (all free).