Use either red or green varieties of rhubarb in your fermented pickles, as well as a wide variety of herbs and spices.  The rhubarb loses some of its tartness during the fermentation process, but not all of it.  Warm, sweet herbs like cinnamon, fennel, and cardamom soften the bite of pickled rhubarb.  While hot spices like chilies, cloves, and bay leaves marry well with the astringency of the rhubarb.