Who made that rule?
The lfs in some areas are full of hero's hybrids. I have seen Andinoacara hybrids, Pterophyllum hybrids and Apistogramma hybrids. And I bet there are many more.
Still Heros X Andinoacara will normaly not cross breed. Andinoacara rivulatus and stalsbergi are genetically not very close to Heros because their ancestors were sepperated by the Andes long ago. But I would not be totally surprised if they still did.