Is a supertransporter considered a different aircraft type than the one it is derived from?
Such aircraft require separate certification. The Beluga XL has just completed its certification process.

From the Wikipedia articles you linked to:  


  The 747 LCF was granted FAA type certification on June 2, 2007.  


and


  Following a total of 335 flight hours being performed during the test program, restricted certification of the type was awarded by the European Aviation Safety Agency (EASA) in October 1995, enabling the A300-600ST "Beluga" to enter service shortly thereafter.


and


  The Beluga XL made its first flight on 19 July 2018, and received its type certification on 13 November 2019.