I'd say that the Guianacara and heckelii are about the same when it comes to food... They even outcompete the buffaloheads.
The tank I described doesn't have discus in it. Just used it as a reference point with regard to Guianacara (at least mine) not being shy when it comes to food
Since I have never kept either of these SA Cichlids let alone with Discus I don't know how well they would mix. Isn't Acarichthys heckelii similar to Geophagus jurupari or Geophagus cf. surinamensis in behavior? I know they are a very showy attractive Cichlid. If they are, the discus and tank are large, 72 inches long/std 125 US gal., they may be compatible.
I really enjoyed my 2 years of combining adult wild discus with G. surinamensis. My Geos were six inches SL which isn't as large as they can get so the "balance of power" might have changed over a longer period of time. Neither fish were very territorial except as one pair of Royal Blue Discus formed but I removed them to their own spawning tank. Healthy Discus don't mind a little competition for food as long as it is an even playing field, so to speak.
If I were starting from scratch I would add the Geos as juveniles and after the discus are about 5 inches in diameter. I think domestic Discus can hold their own as well as the wild fish.
This is one of those projects where it is best to be prepared to be flexible enough to add/remove what does/doesn't work. To me, that's just common sense fish keeping if you are given to experiment as I am. Predicting the behavior of any group of fish has all the limitations of any other time one ventures generalities. They can make a lier out you or all may be bliss.
Here is one of my photos of a Sturisoma aureum attacking Symphysodon discus. The angels are wild Peruvian Pterophyllum scalare.