Discus requiring soft, acidic water is a myth. Some wild fish will show their colors better in soft water and are more willing to spawn in soft water, but tank-bred discus accept a wide range of conditions. The key is that it's consistent. They don't like huge fluctuations in parameters. Most discus hobbyists actually raise out fry/juvenile fish in hard water because the mineral content is better for the bone/fin development. I grow out all my discus in pH 7.6. My current house has moderately hard water but my last place had a TDS of like 200ppm and the discus still grew giant and spawned. Hatching the eggs is a different story -- you often have to soften the water for the eggs to hatch properly. Otherwise they do fine in harder water.
As for the severums and chocolate cichlids, I keep them in the same tap water -- pH 7.6, gH 7, kH 5. I've bred several species of severum in this water (H. efasciatus, H. notatus, H. severus, H. sp. Atabapo, H. sp. rotkeil, etc.). As long as you keep it clean with low nitrates you should be fine. In most cases, the quality of the water is more important than the parameters... within reason, of course. You don't want to try keeping South Americans in Rift Lake conditions obviously.
