I wouldn't be to quick to write it off. Angelfish are cichlids as well. The problem comes in of possible fin-nipping by the cons. If you have an aggressive angel it might work. There is no firm rule of what will/will not work when it comes to con's behavior. The only thing predictable is their possible aggressiveness.
If you want to try this, by all means go for it but keep an eye out for the need to separate. I successfully kept 2 pair of cons with a pair of angels in a 100 gallon (380L) tank. The angels held their own and were not intimidated by the cons. I had other fish as well to help spread out aggression. Sometimes, too few fish in a tank are just as detrimental as too many. You will need to find a balance that works in your tank.