When I calculate what the octahedron should look like assuming that all anion-anion distances are equal I will get two different cation-anion distances:

if all anion-anion distances equal 2R, the cation-anion distances will be 3.5R for four of the anions and only 1.8R (!) for the other four anions.

Does this make sense?

Or am I wrong in assuming that all the anion-anion distances should be equal?

What holds for capped trigonal prisms? There probably there will be a larger cation-anion distance for the capping anions, right? (Otherwise the anions would come too close! Or?)