We study the electronic and magnetic properties of the quinternary full Heusler alloys of the type Co-2[Cr1-xMnx][Al1-ySiy] employing three different approaches: (i) the coherent potential approximation; (ii) the virtual crystal approximation; and (iii) supercell calculations. All three methods give similar results, and the local environment manifests itself only for small details of the density of states. All alloys under study are shown to be half-metals, and their total spin moments follow the so-called Slater-Pauling behavior of the ideal half-metallic systems. We especially concentrate on the properties related to the minority-spin band gap. We present the possibility to engineer the properties of these alloys by changing the relative concentrations of the low-valent transition metal and sp atoms in a continuous way. Our results show that for realistic applications, compounds rich in Si and Cr are ideal since they combine large energy gaps (around 0.6 eV), robust half-metallicity with respect to defects (the Fermi level is located near the middle of the gap), and high values of the majority-spin density of states around the Fermi level, which are needed for large values of the perfectly spin-polarized current in spintronic devices like spin valves or magnetic tunnel junctions. (c) 2008 American Institute of Physics.