Everyone has raised valid points in this thread. One thing to always keep it mind is that the GH is as important or perhaps more-so than pH for soft water fish. And the two (GH and pH) are connected, along with the KH which buffers the pH somewhat.
I won't go into all this, but refer those interested to my article in which I attempt to explain the inter-relationship: http://www.tropicalfishkeeping.com/f...quarium-73276/
Back to the OP's situation. The GH at 6 dGH is perfect for soft water fish and plants. The KH at 3 is not high, so the pH in the aquarium may tend to lower once the biological system is established.
Another factor is what the water supply folks might be adding to the source water to maintain a higher pH. I have to contend with this, so the pH in my tanks does not lower as much as it naturally would with my near-zero GH. It is worth ascertaining if this may be part of the equation, because attempts to adjust pH may not be successful, and the resulting fluctuations are worse than leaving it alone.
The Celestial Pearl Danio will be fine now in your water. So will the sparkling gourami. Neons might, they are a bit tricky sometimes. But you can't combine these three species anyway, or shouldn't, so two out of three isn't too bad.