I am dealing with the same issue for my Chinese MKK. I would agree with Gabor that EW pods should be in pairs. As for the mixing 73 and 77, here is a photo showing such a mix (albeit this is a training loadout). And an Indian MKI actually shows an R-77 on the #2 station - granted, this is a different aircraft version manufactured by a different producer, but still.
Hope this helps.
Alex