Avoid adding PH products. They are a waste of money.
20g may be too small for more than one Dwarf Gourami, they are territorial to other Gouramis.
The issue you may have with your substrate is food may fall down between all the stones and rot. A gravel vac can help, but it may mess up your pattern ?
A DG will enjoy having some floating plants to feel secure. Consider adding some Guppy Grass. It's easy and can grow free floating.
You could also add Java Ferns.
I prefer live plants, and there are some easy ones out there. Plus they can help cycle your tank.
When you say you got an ammonia "treatment" from the store, what did they sell you ?
Without fish, you should be adding Ammonia, not removing it.
Enjoy your tank ! A 20g is a good size.
Here are some plant photos to inspire you
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