Everything I've read about heckelii, and my own fairly minimal experience with them, has led me to understand that they are pretty peaceful with other species, although they can stick up for themselves if threatened (within reason). However, once adult, they can be VERY rough with each other, especially when breeding .
I would be very cautious about adding any other heckelii into a tank with an established singleton.
However, keeping a singleton with Satanopercas I am sure would be absolutely fine (as you seem to have discovered !).