I am going to start making sourdough bread but there are recipes using both tangzhong and starter (levain). Not sure why both needed and how it will achieve given results.
Tangzhong (water roux) and levain are two fundamentally different things.
The roux does not contain any leavening element, it is simply a method to bind water, effectively increasing the amount of water that can be used in a dough and therefore making a light, soft, moist bread.
A levain (sourdough) is the part that is responsible to create the "lift" due to the CO2 of the yeasts' metabolism and, in case of rye breads, deactivate some enzymes that hinder the development of good bread.
You can leave out the roux, but should not leave out the leavener, you need sourdough and / or yeast.