Killis also need colder water for most of their species as well.
And Endler's, they'll interbreed with your guppies as well so that's not too great.
And, fish will adapt to the hard/soft waters. pH is a little different so if you can at least cater to one need or another then that will be great. Another option is to get some driftwood and that will help soften your water in the tank. I also have hard water out of my tap but in my sorority tank I've got four pieces of driftwood; 3 Mopani and 1 Malaysian so it's quite soft ^_^ So as long as it's mostly stable you'll be able to do the Ember's as Tetra's are quite adaptable.