Personally I would go with endlers. All the ones in shops are actually guppy-endler hybrids, but they don't suffer from the same inbreeding issues that guppies do.
About 10 years ago, before I knew about hardness, I had endlers. Males will display to one another, and chase each other but mine never actually made contact. It was all show.
Cories and most tetras need soft water. While they would go with guppies temperament wise, they have different needs.
Hardness, GH, is a measure of the amount of calcium, some magnesium and trace amounts of other metals.
pH is the measure of hydrogen ions.
Although they are interconnected, GH (hardness) and pH measure different things. As a general rule, if the hardness is high, pH will also be high. Though the reverse is not necessarily true - I have soft water but my tap water pH is 7.6 after it's been allowed to stand overnight.
Something else I should mention - it is always a good idea to test tap water for everything so that we know whether something like nitrate is present in the tank because it is being made in the tank or because it's in the tap water we used to fill the tank. When testing tap water pH, we need to test both freshly run water and some that's been allowed to stand overnight. They'll probably be different.