We describe two self-assembly pathways observed in micron-thick colloidal membranes that spontaneously assemble in mixtures of monodisperse colloidal rods and non-adsorbing polymer. In a first example, we study mechanisms by which membrane-embedded 2D liquid droplets acquire unusual non-spherical shapes, suggesting that the interfacial edge domain has spontaneous non-zero edge curvature. These experimental observations can be explained by a simple geometric argument which predicts that the edge curvature towards shorter rod domains softens the resistance of the edge to twist. In a second example, we study the 3D structure of membranes composed of miscible rod-like molecules of differing lengths. Above a critical concentration of shorter rods flat 2D membranes become unstable and assume a bewildering variety of different shapes and topologies. Simple arguments suggest that doping colloidal membranes with miscible shorter rods tunes the membrane’s Gaussian modulus, which in turn destabilizes flat 2D membranes.