You really won't need a test for GH and KH unless you do decide to adjust the parameters. Assuming the 2-4 dH is at least close, this is way too soft for livebearers. You want GH around 10 dH absolute minimum, and 12-15 dH if mollies are included, but 10 dH minimu7m is OK for platies and guppies, but in the 12-15 dH range would not hurt them. There really is no upward limit when it comes to livebearers though extremes are not advisable. I don't know the danio species, but if this is the common Zebra, it will be OK with the 10-15 dH range.
Increasing GH can be achieved by using a calcareous substrate sand/gravel, or mineral salt mix such s those for rift lake cichlids. This is not common salt (sodium chloride) which has nothing to do with GH. Water for water changes must be prepared outside the aquarium. All of this is added work and cost, but that is your decision. With such soft water, you would have a much easier time with species suited to soft water.
The pH will obviously increase with the rise in GH (and KH). That again is fine for the fish mentioned. Just recognize that pH alone is not the real issue, it is the GH. Livebearers must have dissolved calcium and magnesium in the water which enters the fish via osmosis through the cells and this is necessary for the correct functioning of their internal physiology.
