Is it possible to narrow down the scope of tables, fields being extracted to just those that were absolutely required?

Studio gives you the flexibility to scope down your extraction to whatever is required although this depends on the ERP you are working with.

For SAP projects  - Columns must be selected manually before you can map them. While all selected columns will be extracted, it is only data from the mapped columns that is moved to the destination columns. Data from unmapped columns is moved to a temporary space on the database and only leads to a waste of disk space.

For Oracle projects -  all columns are automatically selected, but only those columns that are mapped are extracted.