Focus on the needs of the Application first (i.e. the Client's business data model). Design the Classes and Data Model that the Application needs to the extent of having a working end-to-end Application that use Data Pages that represent the points where your Application will interact with the Systems of Record.
As of Pega 8.5 there is an "Import from Spreadsheet" option available when adding new Data Types to allow existing XLSX or CSV files containing existing business data to drive the new Data Type's data model. This could potentially be used to create *-Int-* classes that model a data type from the Client's System of Record.
Request and Response Data Transforms referenced by the Data Pages need to take responsibility for transforming between the business data model and the System of Record data model. JSON Data Transforms can now do this - in the absence of interface layer Class Models - where JSON messages interface data between Pega and the System of Record (like those typically used in REST integrations).