D-roy was a gillian, not even an adjucha. i see nothing wrong with Rukia being able to one hit an adjuchas with her shikai release either. She is powerful enough to be a seated officer, ones that are authorized to do the more dangerous assignments soul society has. If it wasn't for Byakuya, its possible she could have been a lieutenant. Because, Aizen probably choose the bottom rung Adjuchas to guard this forest.

Renji used his bankai not because the gillian were powerful, he just needed to kill as many of them as fast as possible.