Most fish have personalities and moods much like humans do. They will or won't get along regardless of species. I have one Oscar that hates everybody and everything and that includes any rocks or driftwood in his tank. The only other fish I have been able to keep with him is a 12" Common Plec and a 4.5" Pink Convict. Anything else becomes lunch. These 3 are fish total in a 90 gallon tank. because of it
My other Oscar on the other hand in a 100gallon tank, loves everybody and everything. At one point there were approximately 15 zebra convicts in the same tank that were supposed to be feeders but he "made friends" with them and got all depressed and wouldn't eat when I took them out. Go figure. He now has 3 Copadichromis Borleyi and 2 3 inch female zebra cons for tank mates. The Borleyi 2 females are 6-8" and the male is 8-10"
All my fish are kept in the same water without additives or carbon, I have Malawi cichlids, Malawi Mbuna, SA/CA Cichlids, Indonesian, Thai, etc. They all have the same water but not the same food. They are all thriving with a normal growth and reproduction rate.
Like stated before, 90% or greater of the fish in the US are farm or tank bred. Very few are F0's.As a result it is not as essential to match the water of tank/farm bred to water that an F0 is used to.