Hardness of water first, pH second. If you can keep the hardness of your water under 50-60ppm that will make the Heckels very happy. As long as the pH isn't above 7.0 I find all my wilds do fine, there isn't a lot of difference in colors or behavior between 5.5-7.0 unless you want to get serious about trying to breed the Heckels in which case you want the ph below 5 and hardness to be less than 20ppm.
As for food, I would never train discus to take dried pellets and flakes. Even the best processed, freeze dried food loses 50% of its nutritional value. Why do that to your prized fish...imo feed only high quality frozen food and live food. If you really want to do it, just add little bits of flakes/pellets to what you are feeding them now and slowly increase the amount of dried food until you wean them totally off frozen/live food.
Mark