  We elucidate the interplay between diverse two-dimensional melting pathways
and establish solid/hexatic and hexatic/liquid transition criteria via the
numerical simulations of the melting transition of two- and three-component
mixtures of hard polygons and disks. We show that a mixture's melting pathway
may differ from its components and demonstrate eutectic mixtures that
crystallize at a higher density than their pure components. Comparing the
melting scenario of many two- and three-component mixtures, we establish
universal melting criteria: the solid and hexatic phases become unstable as the
density of topological defects respectively overcomes $\rho_{d,{\rm s}} \simeq
0.046$ and $\rho_{d,{\rm h}}\simeq 0.123$.
