I think it would be good to first of all create the look, the architecture, in your tank. Depending on the way you prefer it to look, you may then decide to add either small or medium size fish. Perhaps some shrimp as well.
To get an idea of how to set up the tank try to google it.
This for instance
Some corner tanks are quite high as well, instead of low and wide.
Then when you're done setting up, take a look at other youtube videos and you'll inevitably see a good number of fish that would be fine in a tank full of soft water.
Personally after trying many fish of various sizes in my 120ltr tank, I have gone small as far as fish are concerned. I have a special affection for rasboras, especially the Galaxy and the Emerald, I can't say offhand if they are soft water or hard though. I'm presently sorting my tanks out into the right chemical makeup to rehome them if needed.
I have a spreadsheet of my fish on my laptop which shows what water parameters each fish and even my shrimp needs. You might find it useful to drop ones you like onto a similar spreadsheet to work things out from the ones you like the look of.