Hardness (GH and KH) measure different minerals.
To make water softer you have to remove the mineral.
By blending your water with reverse osmosis or distilled water (These have no minerals) you end up with softer water.
Of the fish you list perhaps the Pearl Gourami would prefer softer water. They are warm water fish, too. Not so compatible with the others.
Neons, while they are soft water fish, have been raised in captivity for so long they can handle much harder water than their ancestors. They thrive in the cooler end of tropical temps.
Most Rainbows come from waters that change a LOT through the year, from rainy to dry season, and can thrive in a very wide range of water conditions.
Denison Barbs can also handle a very wide range of water parameters, as long as it is cool enough.
Anyway, here is something to try:
Go buy a gallon of RO or Distilled water.
Make a couple of different blends with your tap water.
25% RO + 75% tap
75% RO + 25% tap
On all these, test GH, KH, TDS, pH.
See which comes the closest to suiting all the fish.
These are not really demanding fish, so peat or driftwood for tannins may not be necessary.
If you will post GH, KH, and pH of the tap water we may be able to help with more ideas.