On the GH and KH, don't waste money on a test kit which you may only use once. Assuming you are on municipal water, you can contact the water folks and ascertain the GH (general hardness) and KH (carbonate hardness, sometimes termed Alkalinity) for your water. Once you know this, you will know where you stand, as these values change very little if at all in the aquarium. The article of mine that was mentioned previously and which you read explains the co-relation between GH, KH and pH, and this is worth knowing but unless adjustments are to be made to the source (tap) water, not a big issue.
Some fish do have fairly specific requirements with respect to GH and pH, so it is wise to know what you have coming out of the tap. Selecting fish to suite your water is much easier and safer. And here I'll mention our profiles, under the second heading from the left in the blue bar across the top; data on water parameters, tank sizes, minimum numbers for shoaling fish that need a group, and compatibility issues are included for each species. If the name (scientific or common) in the profile is used the same in a post it will shade to form a link to that profile, as some of the names in your first post did.
When we know the GH we can narrow things down a bit. There are a few issues with the named fish, and bear in mind that those which are shoaling fish need a group which takes up space.
Last, but certainly not least...welcome to Tropical Fish Keeping forum.