Enabling page compression on 1.5TB table
Your plan sounds well except for this one:


  Also, instead of enabling page compression. After Step #3, Truncate
  the largest table, enable page compression and then do a SELECT * INTO
  ReplicaDB.dbo.ReplicaTbl ?


If you truncate a table and enable a page compression, you must do INSERT and not SELECT INTO and at this point you should use TABLOCK to get page compression and not row compression only.

Besides this, rebuilding with compression is much faster that an insert into a compressed table, we have a difference in 3-5 times on large tables.

Have a look here: The Data Loading Performance Guide, section Data Compression and Bulk Load:


  When data is bulk loaded into a compressed table or partition, both
  page-level and row-level compression are generally done at bulk load
  time. However, you should be aware of an exception: When bulk loading
  into a page compressed heap, you must use the TABLOCK hint to achieve
  page compression. If the TABLOCK hint is not used, only row level
  compression is done on the heap. Using page compression on a bulk
  target will generally slow down bulk load speeds – especially if the
  I/O system is able to supply enough speed to saturate the CPUs.