SInce you are planning to keep soft water fish this is really not a problem.
If your pH is 6.4 your tank will likely never cycle (at least not fully) but...
The ammonia you are reading is actually ammonium which is not harmful to fish, but you don't want it to go up forever so add plants to use it, fast growers and floating plants are ideal
Once the plants are actively growing (i.e. you see visible new growth) add fish
Don't forget regular water changes
Here is an old pic of my 20G with plenty of ember tetras I mentioned - this tank has a pH of around 5 and the filter has never cycled. No current pics because I did some serious pruning and replanting today so it looks a bit of a mess at the mo.
FWIW this is what it looked like when I added the first lot of fish - the stuff floating on top was amazon frogbit - the bag of media (from another less acidic tank) was an attempt to introduce some bacteria but it failed.
I am not going to try to count the fish - but there are way more now than I bought