Board Thread:Theories and Speculation/@comment-1872728-20181213131712/@comment-30323490-20181213134537

They didn't transform into Hagoromo or Kaguya.By Sage Transformation they acquired Six Paths Senjutsu, which is something that Hagoromo (and Hamura) had before becoming the TTJ. By becoming the TTJ you get SPS by default, on top of the ten-tails itself. Also the ten-tails at his fullest is Kaguya combined with the shinju. When Madara (and Obito) sealed the ten-tails into themselves they sealed just the Gedo Mazo + (almost) all 9 bijuu.