Your hardness is perfect for your neon tetras but low for your guppies. If you make it hard enough for the guppies it will be too hard for the tetras. I know you like the guppies but it would be much easier to leave the water alone and let them live out their lives. Yes, they won't live as long as they would in hard water but there is no real happy medium with the fish you have.
There is an alternative, but of course it involves some expense - get a second tank, move the guppies into that and 'harden' the water for them. If this is an option we can tell you how to make the water harder.
The other tests look good. Keep an eye on them until you are certain that both ammonia and nitrite are staying at zero. If either show above zero, a water change is needed to get the reading down to zero.
There is an alternative, but of course it involves some expense - get a second tank, move the guppies into that and 'harden' the water for them. If this is an option we can tell you how to make the water harder.
The other tests look good. Keep an eye on them until you are certain that both ammonia and nitrite are staying at zero. If either show above zero, a water change is needed to get the reading down to zero.