API Quick Start is filter bacteria in a bottle. You add it at a double dose every day for the first week and then either poor the rest into the tank or put it in the fridge and use it later on another tank. Try to add it to the aquarium near the filter intake so it gets drawn into the filter where it belongs.
King British Filter Aid is another type of filter bacteria in a bottle. You do not needs both of these products so I would return this one and get a refund for it.
API Stress Coat is a water conditioner used to remove chlorine/ chloramine from the tap water. If you already have a water conditioner (and you should from your last tank), then I would return this product as you don't need 2 different water conditions. If you haven't got much of you old water conditioner left, then keep this but if you still have a bit of the old conditioner, then return the API Stress Coat.
I have no idea what API Stress Games+ is.
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Very Soft water has a GH between 0-50ppm (mg/l).
Soft water has a GH between 50-150ppm.
Medium water has a GH between 150-250ppm.
Hard water has a GH between 250-350ppm.
Very hard water has a GH between 350-450+ppm.
Your water has a medium hardness (243ppm or mg/l), it is at the top end of medium and bottom end of hard.
Rainbowfish and livebearers do well in this water. However, your tank size limits you to small species of fish. You could keep guppies or Pseudomugil Rainbowfish. Other small species of rainbowfish include Melanotaenia papuae, maccullochi, sexlineata, praecox, pygmaea.
There is more info on rainbowfish at the following link.
http://rainbowfish.angfaqld.org.au/Melano.htm