MO-Scheme of SCN- and its bonding properties when used as a ligand
One issue with SCN-, is that it can bind either at the nitrogen or at the sulfur. The way you presented the formula, $\ce{[Co(en)2(NCS)2]SCN}$, suggested that the SCN- ligand is bound via the nitrogen. If it were bound by the sulfur, the formula might be written  $\ce{[Co(en)2(SCN)2]SCN}$. 

SCN- should have a similar MO diagram to OCN-, which is isoelectronic with CO2, so your idea to compare to CO2 is not  a bad one. However, since SCN- has a different symmetry group, you cannot trust that comparison too much. Your comparison to CO is also a good one, since SCN- and CO have the same point group. 

However, what you really need to do is determine the symmetry group for $\ce{[Co(en)2(NCS)2]SCN}$. Then, assign symmetry labels to the SCN- ligands. The MO that is most important for SCN- is, qualitatively, the "lone pair" on the atom pointing at the metal center, which can form a "$\sigma$-bond" with cobalt. I would only consider using those $\pi$ orbitals on SCN- if the complex needs more electrons to get to 18. When you assign symmetry labels to the SCN- ligands, consider them as a pair: i.e. assign a symmetry label to the in-phase pair and then assign a symmetry label to the out-of-phase pair. The in-phase pair should probably have the same symmetry label as the cobalt $d_{z^2}$ orbital.